I make cases for my dios all the time. I go to my local Home Depot, and pick up a couple of 18" x 24" sheets, I think they're like 5mm thick or so. I'm lucky enough to have a table saw, so I purchases a super fine tooth blade made for cutting plastic and thin wood. I measure and cut the sizes I need, and I use a hot glue gun to tack and finally glue everything together. For the top I use a piece of 1/4" wood for the top to save the plastic. I even add 90 degree corner mouldings to the corners to strenghen the case.
I admit that they do not look professional in the slightest, the glue is in blobs from sealing the entire joints of the plastic, so they look really rough, but in my case it's to keep the dust out. You'll notice that all of my bases have rabbits at the bottoms, that's so when I've done making my case, that's where it will rest.
Do not use the glue that's recomended for the plexi/lexan glass, it's almost like krazy glue, and will craze the plastic. I've found that a hot glue gun works the best.
Good luck!
Kelly
"There you go with those negative waves again!"