I think you might just try super gluing the underside of the frag onto the rock. The growth of a montipora frag starts with it encrusting onto the rock around it, this can depend on the species though. So to allow for more encrusting the frag should be placed on the rock so that the most surface area possible is in contact with the rock.

Also, depending on the size of the frag this may not really matter. If it's a small frag it may encourage growth but it could also affect the way you wanted things arranged. A good idea that many people do is to glue the frag onto a small piece of rock to allow you to move it around untill it outgrows the little rock.

Hope this helps

Aaron