I saw this product on Amazon (see pictures) and really liked the design and thought it would be easy enough to replicate. This is a fish hideout for the corner of a fish tank. The high lips on the opening allow it to be buried under the substrate and the open sides allow visibility if placed against the front of the tank. This 3x3 model can be scaled to accommodate tiny shrimpies or shy loaches.

Be sure to print at 100% infill if you want it to sink! Also, support material is inevitable on this one, sorry.