We built this frame out of 1/2 inch PVC pipe. Since we have an acrylic tank, we glued the frame so that we could split it into two smaller sections which could pass through the access holes in the top of the tank. Holes were drilled in numerous places in the frame, which served two functions: (1) they allowed us to pass cable ties (see below) through them, and (2) they prevent the formation of anoxic areas inside the pipe
We decided to make "shelves" out of the PVC. This proved to be very helpful because we could give the rocks a "staggered" look. Note the holes drilled liberally through the PVC. We did NOT put the frame as far back as is shown in this picture -- we left room to put hidden powerheads behind the rock.
With the frames in place, we used cable ties to attach rock to the frame. Cable ties were passed either through natural holes in the rock, or through holes that we drilled. Drilling was with a cordless drill and a masonry drill bit. We found that it was extremely easy to drill natural rock, but the few pieces of man-made rock that we had from our 55ga aquarium were impossible to drill using this method.
As we continued to build from the bottom up, we continued to attach rock to the frame and to each other with cable ties. The cable ties were so effective and easy to use that we decided NOT to use the marine epoxy we had also purchased. The use of the frame and cable ties allowed us to build a very "loose" structure with plenty of overhangs, holes, and caves for fish and other sea creatures to enjoy.
Once all the rock we had was in place, we added live sand, invertebrates, and fish. Note how loose the structure is.
On subsequent takedowns, we also added sheets of plastic fluorescent light diffuser grid (the kind with the little open squares), spray-painted black. This allows more points of attachment.