Well, PHs usually come with a "holster" attachment, if you may, that also come with little suction cups (the little round plastic thingies that attach to anything flat). If for some reason your PH
package didn't come with suction cups, you can always go to an LFS
or Petsmart for it.
Undergravel filters usually have airstones, as far as I remember. They mostly pump air into these cylinders and the bubbles travel back up. That causes a suction from under the gravel. Now, UGJs need powerheads.
I just want to clarify that because early into the hobby a couple of years ago, I thought airstones were powerheads. It's just my newbie blooper.