Vypyrz must have missed where you said no litter boxes. Thats what those basically are. You could make it out of clay and seal it but you would need one really big kiln. Maybe you could find a art teacher, once school is back in, and they could get some students on it. Something like that would weigh a ton though.

Maybe check around with some landscape places for big fake rocks. Don't know what the inside would be like or if its snake friendly but I'm sure they could tell you. Cut the bottom out of it and you'll have your hide. Its not mosiac but its naturalistic.