I wouldn't use a rubberband myself, that's not to say it won't work. Really should use glue, don't be afraid to glop the crap out of it, turn your powerheads off when you mount it. After you glue it submerge and squish against the area you want and hold it there for a good 30 seconds until it grabs hold, wait 15 minutes or so before you turn your powerheads on just ot make sure it holds, and give it a gentle nudge with a finger just to be sure at the end, if it doesn't work try to put some glue on the glue and try again.