Another way to build a small enclosure is to use a broken slate tile. I went to my local building supply store and bought a single slate floor tile that I broke into pieces. I used silicone much the same as NavyScuba to glue the pieces together and had this end result for my
kribensis to use as a shelter. Because it has no bottom to it, I just sit it on the gravel surface.
I think the most creative one that I have seen is a piece of
PVC tube cut to length and covered with aquarium silicone. The silicone is just rolled in the gravel that you are using in the tank and the result is a tube that blends right in. These are used by plec breeders.