This works for me every time--
Find a piece of rock/rubble big enough to support the frag but small enough to position easily.
Drill a hole in the rock about the size of the "stem" of the frag, fill liberally with superglue gel, and stick the frag in.
You'll find this method leads to rapid encrusting, and allows you to move the frag/colony around a bit to find where it's most content. Looks much more natural that a big wad of epoxy--and keep in mind epoxy will sometimes RTN the frag as it gives off heat while curing. I only use epoxy to fix down the rubble the frag is attached to. A dab of super glue gel on either side of the epoxy makes it hold much better underwater.
John