Hi Jack i use wood blocks cut up in to 1.5"x1.5" x what ever lengh you desire and i cut a 24mm hole for the cave aspect, you can use Oak, Fruit Tree woods, buy some bogwood and just drill a hole in that, i got hold of a old oak front door cut it all up in to the blocks then drilled the holes and sanded it down it workd a treat and didnt cost me anything and the plecos love it