2 part epoxies will drive yoru skimmer insane and can burn frags, killing them. Just use palin old super glue gel from any hardware store. You can use it underwater. I do it every week. For a frag though I'd suggest gluing it to a small rock first and then glue the rock to the actual rock work. That way if you change your mind or somehting happens you can easily pull the small rock loose and move it.
Remove a tiny bit of rock for a day or so. It's always a good idea to have a few pebbles worth lying on your sump ready to go.
Get a small container of tank water, any small amount should do.
Put a dollop of glue on the rock.
Pat dry the bottom of the frag quickly and carefully hold it to the glue for abotu 5 seconds.
Submerse the rock in the container for a few seconds, remove, dip, remove, dip.
That will quickly set the glue and frag.
reach into tank with rock and glue. When in position put the rock where you want the frag. Put the super glue under the rock. Carefully push out some glue between the rock and the base. Hold in position for about one minute. You should be good to go with some practice. Any extra glue can easily be pulled away when set. Coraline will cover any glue that does not come loose. It is best to not be stingy with the glue. Plan on one small tube per frag. Buy only the gel, not the regular stuff. teh 2 best brands, in order, Loctite gel and Durco.