Custom build questions

I'm working on making my first custom. I've done a fair amount of research but still have some material concerns before I begin. For wood, depending on price, I was going to use melamine or smooth plywood. I was gonna use wire mesh for part of the enclosure top and plexi glass or some sort of clear plastic for outward opening doors.
My concerns come from sealants for a few things:
-I've heard mixed reviews for polyurethane. Just from the fumes alone I'm worried, but I don't know if paint will hold up on the inside either
-I wanted to do a carved foam hill, but I know that also needs to be sealed. I haven't seen much for beardie safe foam sealants so I'm lost there
-Finally, caulking. I want to caulk around the perimeter to help keep in heat, but i have the same concerns there as I do with polyurethane
Do you guys have any trusted brands you could recommend? Much appreciated

I'd go with plywood since it's quite a bit lighter than melamine coated wood. Using polyurethane to seal it is fine, just give it plenty of time to cure. Using a non toxic caulk should work. I'm not sure about sealing foam though as I've never worked with foam before.

If you shape the foam, then cover it with grout you can purchase non toxic sealers online. Just Google "no toxic grout sealer"