I have had luck welding the plastic with wet supeglue. While the glue is wet light it on fire and it will melt the plastic back together. I usually clamp the top tight while I do this. I have never done it on a bow front tank though. Just lots of 55 gallon tanks.
I sent him the advice from here and after considering that and talking to my uncle (who is good at everything) here's his result. If he had to do it again he'd put the two a bit closer to the center, but so far it seems to be perfectly stable.