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
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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?