I have used silicone and a hot glue gun to attach gravel or rocks to the pipe. I have had the plecos not use the tube with out knowing why. Time has thought me that if the diameter of the pipe is too big they won't use it. If the pipe is too long they won't use it. Also if they can't see the back of the cave they will not use it. I have found if I make the inside of the cave a lighter colour they will use it. I use a 1.25 inch pipe that is 4-5 inches long and put tan coloured epoxy in the tube. The end also must be closed.
I hope this helps. Travis.
Yes you need to close one end of the pipe for plecos. They will go in head first and back out when they needed. That is why the diameter of the pipe is so important. If they feel they will get stuck they will not go in.
T-bone is also correct ABS flotes and PVC sinks.