What I did in my Aquapod 12 was to use superglue gel and Aquamend epoxy (from Home Depot) to join my LR together. I'd take the 2 pieces of rock I wanted to attach, put a ball of epoxy in between them and make note of where the epoxy touched the rocks. I then coated those areas with superglue gel, and pressed the epoxy in place. The point is to have the superglue attach the rock to the epoxy, and to have the epoxy take up the irregular free space. I found that it worked very well.
In my case, I made one base out of 6 or 7 pieces of LR, and placed some other moveable pieces on this base. That way, I can customize as I need. You could also make a 2 or 3 piece base that can be easier to get in and out of the tank.
I don't think this would be so permanent that you wouldn't be able to disassemble it if you wanted to down the road.
The only thing you may want to do is turn off your skimmer if you have one. I don't have a skimmer, but I read that the epoxy will make the skimmer go nuts. Other than that, everything I've read and my own experience shows it to be safe.