Anything with an entrance 1" x 1.5" or smaller
They dont need a whole lot of room. People actually glue the base of flower pots together here and drill holes in them. Or silicon slate or tile together. They like having multiple caves the female will enter one and the male will trap her in there.
Try to create as many caves as possible, even naturally just the way you set up the tank. I had a pair of mine breed in a piece of dw. BNP = very easy to breed