Started a rock wall for the background based on the DIY information posted.
I would have preferred to use the blue core solid foam instead of the white stuff but our home centers don't seem to stock it. What a mess carving the white stuff!
After one coat of concrete:
Test fitted the background last night. Learned very quickly that the concrete on the backround will EASILY scratch the glass badly when installing. So, what was going to be the front of the aquarium is now the back. I'm glad I'm learning all this on a free donor aquarium instead of a $$$ acrylic show tank. Need to inspect the rest of the tank during daylight tomorrow and see if I did any more cosmetic damage. From what I'm spend on the equipment already, if it's bad, another $100 for a LFS 55g tank wouldn't be bad as opposed to always staring at scratches.
Going to put the 3rd coat of concrete on the background tonight and it should be dry and ready to install on Sunday.