I found this vid of someone making foam rocks for a backdrop, maybe it'll help.

https://www.youtube.com/watch?v=Mpko5rxM9pI

Also, the color right now for the rocks is very bright brown and shiny, which makes it seem unnatural. I'm sure scoring/sanding the foam will help with the shininess, and maybe a more grey-toned paint could help. In the video they mentioned using a sand-mixture to give a more matte and textured surface. You could do that and still have a sealant layer on top. This is such a cool build and I feel you on wanting to blow all your cash on treating your snakes lol. Texturing and shaping the foam can help pass the time until payday.