Some people use putty, some people construct the entire thing outside the tank, drill holes, put supports in with PVC, then set the entire setup into the tank and fill with water.
It depends on what you are going for.
there are a few people that make man made live rock. IMO find one of them and have them make the 'arch' for you and avoid ANY stacking or building! You just send them a drawing and dimensions and WALAA! They lurk the boards just like you and me, just search around.
Good Luck!
Jessica.