How do you build PVC racks so that they can't be seen? I Have a 92 bow and would like to build up the back two walls. Is it just a matter of trial and error? Most of my rock is softball size.
Use PVC for the legs of the structure and cover it with eggcrate. You can place the rocks on top and in front of the structure.
If you really want to hide the structure further you can apply "the great stuff" polyurethane foam and coat it with epoxy sprinkled with aragonite. In time it will get covered with coraline and blend.