+1 on the Drylok. MUCH better than using concrete. It doesn't mess with your Ph and there's very little cure time compared to the concrete. I used it on the 3D background I made for my 135 gallon and it was great to work with. But in the end I ended up not using the background because I didn't want to take up too much space (on the depth of the tank) because I was going to have some very large fish in there and didn't want to have to tear it out in a year. I had painted the back of my tank black first and that is what I ended up with.
Good luck on your project.