How to make a rock wall for my hermit crabs?

I’m not sure if I can add links to pics but I do have them. I need to replicate something I saw at a petstore. The made three walls of the aquarium climbable by pressing small gravel into some sort of epoxy, cement or adhesive and letting it set up. It is strong enough to hold large flat stones as ledges. I can’t determine what they used but I need a similar substance that would be non toxic. I was thinking of using plexiglass as the base. I’ll try to come back with photo links in case it’s allowed.