Mine was a bamboo cave about 6-7" long, maybe a 2" opening. It was closed at one end in the beginning, but the end has deteriorated so it is no longer there.
I've had it about two years and really should replace it. Just hope the plecos appreciate a new house as much as I would!

I had three spawns very close together. As I mentioned, I didn't save the 2nd one that was kicked out of the cave, but the babies from the first and
third did great and most have gone to the LFS - about 5 or 6 left to catch, a couple of which are in another tank with lots of plants, unfortunately.
Haven't had another spawn for a couple of months now.