This an old pic I found of a false bottom I once made for my 90g cube. If my memory serves me correct, I used a thick sheet of plexi glass. Put on a layer of reef safe silicone and then encrusted it with 1mm to 2mm aragonite. Lots of aging and rinsing after.
Worked and looked pretty good. Did collect detritus but could be sucked out during water changes.
Ot at least I think it did.
Was a long time ago.