Well if you've never worked with fiberglass before, I'd recommend playing with it, maybe building something simpler before diving into a large background... but if you're confident, my progress should paint a pretty easy to follow picture.
I'm taking a mold of the two back corners of the tank (or a partial mold anyways.) I've made this fairly heavy, because it will want to be really flimsy when I remove it, and I need it to retain it's shape.
After I take the two molds, I'll put them aside while I fnish the tank, and while the tank is cycling, I plan to build the shape I want for the front part of the background using foam board/expanding foam. I'll then mask the foam and fiberglass over the form (FG resin will eat the foam if I don't mask it.)
Theeen, I will remove the FG from the foam form, and scrap the foam. I'll trim the FG shell to the right shape, and join it to the rear mold with epoxy. On top of this shell I"ll do the concrete/paint/moss thing. Then seal with epoxy.
The reason for the extra trouble of the Fiberglass, is I'll be left with a HOLLOW rocky structure, so I won't displace much water, and I can make several openings in this structure, so it's a large cave in the rear of the tank for shy fishes to hide in. Because of the way the back of my tank is shaped, having the back of it be a partial mold, will allow it to kind of pop into place, and be held down with some rocks (FG is heavier than water anyway). This way I can quickly move it if someone gets stuck or I need to clean back there.
Anyways, I'm procrastinating... I need to go work out