I was at the local PetSmart looking for meal, erm, ghost shrimp and ran across this little item. It was on the end cap of the reptile aisle and labeled as a Little Crab Castle for hermit crabs. It is smooth with no sharp edges and has four holes on the top. It would be a fun project if you are so inclined.. maybe cover the exterior with some sort of gravel or stone or even substrate. I would imagine it would take a good amount of aq sealer but still worth a shot don't you agree? You could also anchor a plant of some sort to the top too.
Pics attached for your viewing enjoyment. I am open to suggestions, I want to experiment and try something interesting.