Looks nice so far and a good use of a busted tank. I'd be a little worried that the great stuff might bust the back up more while it's expanding and curing, how long has it cured for and how bad was the damage? I hope it holds for you.

I don't know that I'd trust silicone to seal to a piece of plexiglass and be water tight. Can you get a piece of glass cut instead of plexi?