I use small mag floats . I have two mag floats with caps on them . One on my overflow and one on the back glass. In my last tank I glued to different cours two the overflow with frag glue and it worked well. They were two small pieces though.
Heres a stag glued to a frag plug to the overflow.
Power heads are a good surface to glue frags or caps to as well.
RIO 1100 with cap
SEIO 1500
You can also make the mag floats into frag trays by drilling 1/2" holes into them. You do though want to put some silicone in the holes to seal up the magnet again.
I will take a few shots tonight of the caps and post them.