My smaller adults really seem to like the plastic coffee cans. And it's easy to cut a hole in it and melt the edges with a flame. Some I leave on their sides, others I leave upright, and the snakes perch on top of them.
And if you like to get creative, you can take the butter or yogurt containers (or a slice off the bottom of a soda bottle, etc.) and glue rocks or make and glue putty 'rocks' over the outside to make caves or crumbled/ruined buildings for them to hide in.
Just be sure to use a glue that will be safe when it's cured (aquarium sealer is popular) and use a flame to gently melt cut edges so there's no sharp spots. Plus you can paint them too! (non toxic paints)
For the techy Geeks, there's a scalable 3d model of Castle Grayskull (as a reptile hide) for free on some of the 3d printing share sites.