I don't know of any really good sources for display case plans. (P.S. When I originally typed this I didn't know Stinger was typing a response at the same time. I'm sure his drawings are excellent.) I do have some opinions on the subject; if the membership will forgive me, I'll take this opportunity to air some of them.
There are essentially two materials for making the transparent parts of a display case: glass and plexiglas (more properly "clear acrylic"; plexiglas is actually a trade name, but it's in almost universal use.) Each has its advantages and disadvantages.
Advantages of glass: it's cheaper, it's rigid, it's hard to scratch, and it doesn't offer the problems of electrolysis that plexiglas does. (See below.) Disadvantages of glass: it breaks easily, is difficult to cut to precise dimensions (unless you have considerable practice and skill with a glass cutter), and making the joints at the corners of a glass case really requires some sort of metal or wood structure. (There are adhesives, such as aquarium sealant, that work on glass, but a case made that way looks pretty ugly.) For the latter reason, modern museum exhibition designers don't like glass cases. Those people contend that the corner moldings block the sight lines of the observer, and turn the case into a piece of furniture that attracts more attention than what's inside it.
Advantages of plexiglas: it's more durable (to bust a piece of plexiglas takes real determination), normal mortals can cut it (either with a table saw or via the scratch-and-snap method), and it can be glued at the edges. Disadvantages of plexiglas: it's expensive, it sags (a potential problem in the top of a big case), and it triggers electric reactions inside the case. I'm not enough of a physicist to explain the latter, but it can be really problematic - especially if the model has any lead in it. That problem gets worse if direct sunlight comes through the plexiglas. (Letting sunlight fall regularly on a model is a bad idea under any circumstances, though.) Plexiglas also seems harder to keep clean, and it scratches easily. (By the way - in cleaning plexiglas never use Windex, or anything else with ammonia in it. Ammonia is a solvent for plexiglas; use it often enough and the plexiglas will start turning grey and blurry. Clean plexiglas with diluted mild dishwashing detergent, like Palmolive or Ivory.)
Take your pick of materials. The last case I built has plexiglas panels in a wood frame. I don't care for the appearance of glued joints in plexiglas; they're fairly easy to make, but they look kind of amateurish. The big museums pay through the nose for fancy methods of bonding plexiglas so the corners are nice and shiny, but those are beyond the means of most of us.
I've seen quite a few case designs in print, and frankly I don't like any of them. Most of them require that the vitrine (the transparent part of the case) be lifted over the model and set down on its base. (I recently bought a book about showcase construction; it included plans for building a ship model case with a removable TOP. That strikes me as even dumber. Apparently you're supposed to dangle the model by its mastheads to get it in.) Especially in the case of a big model, a vitrine that lifts off the base is an invitation to disaster.
I've been building ship models for 48 years, and I spent three years as a curator in a maritime museum.
Almost every incident I've heard of in which a ship model has gotten wrecked or damaged, either in a museum or in private hands, has involved taking the model out of a case or putting it back in. Somebody got a little careless, or nervous, and banged the vitrine into some protruding part ot the model.
The most ridiculous exhibit case I've ever seen is in the "Great Hall of Steamships" at the Mariners' Museum, my former employer. A 15-foot-long model of the liner President Hoover sits in a case with a steel-framed glass vitrine. The vitrine weighs about half a ton. To open the case requires two custom-made steel straps (which are held to the bottom of the vitrine with bolts), two chain hoists hooked to the beams of the ceiling, and three people - two on ladders working the chain hoists and a third giving instructions. The one time we opened it during my time there, the director of the museum happened to walk in while the vitrine was on its way back down. The spectacle practically gave him a heart attack; he had to leave the room for the sake of his health.
The best way to make a case, to my notion, is to design it in such a way that the model
comes out one end. Several other cases at the MM work that way. They make it possible for one person (though in the case of a 15-foot model he'll probably need a helper) to take the model out. (One of those cases with removable ends did have a problem: even after lengthy inspection we couldn't figure out the model was supposed to come out. I vividly remember the day I had to fix a busted rigging line on it - lying on my belly on top of a step ladder, with an assistant holding my ankles lest my center of gravity shift and I land spreadeagled on top of the model.)
I built my last case with a removable end. To open it, you set it on a table with one end overhanging the edge. Twist two latches on the bottom and the end slides downward. Then move the case to the center of the table and slide the model, complete with baseboard, out. No busted masts, no frazzled nerves, no cuss words. Figuring out how to make it took some time, but the actual construction process actually wasn't hard. The only major tools it required were a table saw (to cut both the wood parts and the plexiglas) and a small router (optional, to make decorative edges). I used cherry, which I think is a beautiful, workable wood for the purpose. Mahogany and walnut are nice too, but my first choice is cherry.
If you do decide to make the framework out of wood, try to avoid miter joints. The population of this planet is divided into two groups: those who find it easy to make miter joints and those who don't. The latter are in the majority, which includes me. Even if you're good at miter joints, though, they're among the weakest joints in woodworking because they rely on end grain (unless they're splined or otherwise reinforced - difficult on small pieces). On my last case I managed to get by with only four splined miter joints. They're in the framework of the top, and the pieces that make them up are glued permanently to the sides and ends, so the miters aren't under any stress. The case I built before that one has miter joints all around the base, and they're falling apart. One of these days I'll replace that one with a no-miter, ship-comes-out-the-end version.
One other point. Give some thought to where and how the model will be displayed. Ask yourself whether all four sides and the top really need to be transparent. In many household situations the back of the case will be against a wall. If so, consider making the back out of wood, and painting or otherwise finishing it in such a way that the color makes a nice contrast with all parts of the model. The rigging of a sailing ship probably will have both light- and dark-colored thread in it. The model will be much more impressive if both stand out against the background.
Another consideration is height. If the model is to be displayed in a house you may not have much control over that, but if you do it's worth thinking about. To my notion the ideal way to display a sailing ship model is with the deck a little below eye level. That causes the observer to look down at the deck and the hull, and up at the rigging.
Those are some personal thoughts. Hope they're of some use. A case is worth a good deal of time, effort, and thought. A lousy case can wreck the impression made by a good model, and a good case can make a mediocre model look good.