I have two fish tanks, a large one for fish and a small one for dwarf shrimps. I want to minimize maintenance tasks, but to put dwarf shrimps in the large fish tank runs the risk of losing them to the fish. As such, I built a small acrylic box to keep the dwarf shrimps in the large tank in isolation.
The box has to have small holes so that water could flow between the box and the tank, so the shrimps can enjoy the benefit of the water change. The complicated part of this project was to find metal hangers that fit nice ly with the dimension of the box. However, this is not terribly difficult.
The drawback of this solution is that you get a lot of debris inside the box that it is hard to get rid of because the box is quite small to work with. Aside from that, it works quite well.