I used PVC toilet flanges and aquarium putty to build my structures in my 34g (pic on my avatar). The toilet flange is a nice thin PVC rod so don't have to use too large of a masonry bit to drill the rock. Bought some dry rock from Marcos to take my time to build the back structure, then filled in the front, lower structures with live rock. I cut a piece of cardboard the same dimensions as my tank, and tried out different rock structures before drilling, connecting with flanges, and placing in the tank. This allowed me to avoid any possible unsecured rocks from falling against the glass.