I haven't tried that, but I do use a sea urchin skeleton as a frag holder in the sand. It is low profile, has a hole that is a good size, and I figured if it encrusts it would be easy to break into frags.Has anyone tried using something like this as a way to hold frags?
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As for attaching things to the back glass, ocean wonders creates disks for this https://www.bulkreefsupply.com/6-pack-mag-disks-oceans-wonders.html but I hear the magnet is weak. I bought these in the last month but haven't used them yet.
I did make my own magnet holder though out of macro rock that I sanded flat on one side and glued in several ceramic magnets sealed in aquarium silicon. I used this to hold a shelf for my monti cap that has now taken over a whole corner of the aquarium.