The wax paper idea is pretty good! I'll have to try it.
1.What I've done in the past is use Minwax's Acrylic Gloss Polyurethane. It looks like Elmer's Glue going on, but dries hard, and seals well.
2. 1/4" plywood sounds a little thin. I've not had warpage problems with 1/2" or 3/4" stuff.
3. I am learning to stay away from the unfinished plaques in the stores. Pine and basswood are just too soft, and warp when I look at them wrong. :) I go to Home Depot or Lowe's and buy a couple of pieces of oak "shelving". These are boards that are planed and sort of pre-finished. I fasten some plastic window screen material to the base to give the Celluclay some tooth to grab onto.
4. Celluclay needs to be mixed as dry as possible. I use a little water (<1cup), white glue and a few drops of Liquitex acrylics (to add earth color) mixed well in a cool whip container, and then pile in the Celluclay. If it looks like there's not enough water to mix all that together, then you have the right ratio. A little water can be added as needed, but start off DRY.
5. The thinner the Celluclay is placed on the base, the better. If you want a thick base, glue down some styrofoam on your base and add the Celluclay over the top. This way, all that moisture doesn't come into contact with the wood.
a. If you do need to build up Celluclay without the styrofoam, do it in increments. Let the bottom THIN layer dry, then start adding a little more until you get it where you want it.
6. As a last resort, change base material. I've seen some good stuff mounted on ceramic tile flooring material.
Gip Winecoff