I notch clay flower pots. For Africans, I stack the pots along the back to build a structure, then cover the pots with rocks so it looks like there is just one big pile of rocks. It takes up less volume of water, is lighter than all rock structures and provides more hidin' spots and escape routes than a solid pile of rocks.
For breeding caves, a single pot with a notch or with the bottom drain hole enlarged to allow the fish entrance works well. I use pots from 2" to 8" for different species, and they're also cheap. IIRC 4" pots were 80 cents, up to like $2.00 for an 8 incher.
WYite