If I were to do another background I would try the drylok just for change. I didn't mind cementing my work. I didn't have fish in my tank and wouldn't have the tank ready for fish for a couple of months as I am building a new stand. So the cement worked great for me. I had time to wait for the curing process.
I think you main issue is going to be mounting it to the back glass. If you don't use magnets you are going to have to drain the tank and wait 48 hrs for the caulking to cure before any water can go back in. Good luck. It's a great project to work on.
Any other less..... Buoyant solutions? I love the look, but any ways I could achieve it not using styrofoam? I think I'm going to go with the cement shell idea, and just burn the styrofoam out after... But if someone has another solution I'm all ears!
If you really want to get rid of the syrofoam spray acetone on it from a spray bottle. It will melt away. Make sure you leave some on the sides and top and bottom so you can attach it to the back and it won't allow your fish to get in behind it.