I just got done building a wood enclosure. I waterproofed/sealed the entire thing with Thomson's Water Sealant, then left it outside for a week to dry. I then put a 4x2 piece of Reflectix insulation on the bottom, laid two 2x2 pieces of Flexwatt on top of that (side by side), then covered the whole thing with a 4x2 piece of plexiglass and sealed the egdes with 100% silicone caulk. Give the caulk about 3-4 days to cure, and it'll be ready for the new kids to move into.

With the two seperate pieces of flexwatt, I'll have a nice thermal gradient for the kids to thermoregulate on.