I have the 20 and the 5 hole frag rock. I love them as well. I have also seen "dougnuts" that are a clay ring with a hole in it and lets your corals encrust a large piece.
Favites and chalice should be fine really to grow naturally on the sand. They both create their own skeleton.
I also attach small pieces to medium small rocks so that I can place them or move them as I like. Mainly for zoas.
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They are growing without slowing... but, they seem to be growing underneath, rather than across the sand bed. I am sure it is due to the snails tipping them over every third day or so. So, they never get the chance to attach.