If you Google hides/basking spots made of foam, you will get some great ideas. I'm starting to make one for a bearded dragon using sheeted 1/2" foam board. Cut/shape it however you want, stack sheets together with glue/toothpicks/dowel rods, then once you have the shape you want, cover it with grout and then a safe sealant. This has been done for backgrounds and all sorts of ledges/hides/ramps, etc...