I use a carbide drill bit to make small "wells" in whatever I attach frags to. Really only needs to be a small depression. I keep a bunch of different size "collars" handy that will surround the disc, plug,rock, or tile squares. The collars are just various size pieces of pvc pipe and vynil tubing. I just put a dab of glue in the well, sit it inside the collar, drop the frag into the well, and let it sit in a container of tank water until it sets. Using the right size collar will keep the frag erect on the plug. Eliminates whatever type glue you use from running, and you don't have to hold the frag in place until the glue sets.