If it's an aquarium just attach on the outside. Anything will do, but foam board or styrofoam will help moderate temps. If you already have opaque walls, the snake probably won't notice, whatever you do, but you might enjoy the scenery more. You could paint a scene with non-toxic paints, or maybe attach a pre-made background with that putty stuff used to hang posters. The snake will love going behind the background just to annoy you. There are all kinds of websites devoted to the carving, coating, painting, and planting of expanded foam to make realistic rock outcrops, or whatever your imagination can come up with. Google will give you more ideas than you can deal with.