I used some large diameter ABS pipe cut into slices with holes in the sides of the slices to allow some measure of circulation to support my rockwork. Next time around I will likely cut them into half slices like this & silicone them to the bottom.
The pipe I used on the current tank kept floating up, turns out it is buoyant. Will take a bit of planning, but now is the time to do it, with an empty tank. I suppose one could simply scatter a few of these all over the bottom & play around with setting up the rocks. If they get exposed later by sand movers, the black won't be quite as obtrusive & will eventually be covered in coralline. Strategically placed to support each rock, they shouldn't be easily exposed. If they really become an eyesore, easier to remove than a large piece of egg crate. Large diameter white PVC pipe could be an option as well.