That's an easy fix, chubb. Bog is your friend here. Just go crazy with the bog, then sand it into something more-or-less approximating the original shape, and bung a cf slider on it. Job done!

I have a tank here the same that someone has tried to repair 'properly'. They've cut a largish square out from the underside to get a slide hammer or something in there, then punched the tank back into shape and welded the square hole back up. It's a shitful job!! The welded bit looks terrible (admittedly it's underneath, out of sight), and the 'finished' top bit is full of small spiky sticking-up bits from the hammer! Woeful. I will now have to sand metal down to make the tank surface smoothish. A container of bog would be much preferable.