I took a rock or two and stretched it and cut it in half. Then I arranged them so they merged and added some holes. The goal was to have some natural looking rock like structures that will fit in the corner of a tank and let the fish explore/hide, and possibly nest, as some will not lay eggs in the open. The back is open so as to minimize the loss of water space.
This dirty tank has a bigger corner pass through.
The idea was to make it look like solid rock but not displace the volume. I used aquarium charcoal and streaked it to make it look more rock-like, but the algae should chip in for you.