Why not drill them and pin them with plastic rod. A lot of folks have gone that route and it works fine.............LR is easy to drill, and on most lr you don;t even need a masonary drill bit......1/8 or 1/4 diameter holes and plastic rods/pins would easily hold it in place. I have three pieces of rock in my one tank pinned with plastic rods that are 1/8" in diameter to form an exctended overhang / shelf. I used the PVC plastic weldoing rods from Harbor Freight to cut the pins from......about $3.00 for a pack of 1# of assorted plastic welding rods 12" long and 1/8" diam made out of ABS, HDPE and PVC, all of which are suitable in a sw setup.