Foaming the back of the tank directly would work if you can lay the tank on the side, otherwise the rocks would slide down. But I do have a friend with the same tank as mine who actually did foam directly on the back, not sure how he managed to get the rocks attached though.
Anyways, here is the continuation of my project
It's time to fit the panels in the tank
Well, they "grew" again and had to be trimmed once more. But worse, the right piece that covers the overflow and about 12" of backwall will not fit at all anymore, the radius I made for the overflow is totally off and the angle at which the straight piece is attached is wrong also. If I force the round piece around the overflow, the flat piece is about 2 inches from the backwall, if I force the flat piece against the backwall, the round piece doesn't fit anymore. No...I don't want to make a new piece! So, we sawed it in two, fit it in the tank and refoamed it in place.
Trimming
That's how much came off
There where some rocks in the way too
Here is the piece that wouldn't fit, cut apart
Well, they are in and getting siliconed to the back wall
Big gaps between the pieces but I can fix that