Terracotta flower pots, or a coconut cave. Just cut a coconut in half clean it out and boil the living sh*t out of it. Literally, I usually have to boil mine 3 times for about 10 hours in total to get rid of all oils comming out of it. The make awesome breeding caves though, really nice for planted java moss or windelov or java fern on it.
There are asian based companies that sell specifically made breeding huts too.