When I decide the semi-permanent place to put an acro or milli frag, I cut the stem off of the plug, blot off excess water with a towel, place a couple drops of ic gel where the stem was, place a 1/2 size ball of quick curing epoxy such as Aquastick by Two Little fishes, then one last small glob of ic gel on the ball of epoxy before placing it on the skyline.  It may still be removed with mild difficulty, but not by the hermit crabs!