What sealant should i use on a fake rockwall. reptiles custom cage.?

hello, i just recently constructed a rockwall from styrofoam boards. i carved the shapes and everything and covered it with 3 coats of joint compound and sprayed it with grey paint. now i just need to seal it because its going in my chinese water dragons cage. i will need a sealer that can withstand alot of humidity and something that wont poision my lizard. any info is appriciated!! ohh and i heard that minwax waterbased polycrylic would work, is this true?