I recently embarked on a similar venture with a 30g I found. I replaced one broken side with Plexiglass and tried the same for the back. After the tank was half full of water it looked like a Bow front. So at this point I'm thinking of just using the original glass backing(cracked in bottom corner, reason I was going to replace) but I want to drill holes for my overflows like reefnewb.
Do you think it is feasible to just score and crack off about 3" of the already broken end, replace with plexiglass with drainage holes, and maybe create a built in corner overflow by covering the corner with another piece of plexiglass???
I hope I made sense....