The inside of the tank had been coated with
"cream" ( an anti-leak epoxy coat) a few years ago,
and this proved to be very difficult to remove. We tried sandblasting
it but that didn't work so we blowtorched it and then sandblasted
it again. That did the trick!
We cut a rectangular piece of 1 mm sheetmetal,
and using a hydraulic press, a round 50 mm piece of solid steel
and some pieces of wood (!) we shaped a new tunnel.