I used Duratank to make 4 x 1450 litre rubber bag tanks. The bags side in smooth walled/bottomed 18mm plywood boxes - the brown coated stuff.
Duratank will be able to include any form of BSP fitting required.
As long as you can cut a hole in the tank to unroll the bag and suitable holes for the various pipes, it should do the job.
Our ageing 900l water tank has decided to leak from somewhere. It is difficult to get at and will be very difficult to remove. I vaguely recall some people using tank liners, something like a custom made rubber 'inner tank'. Alternatively we will need a new tank or a way to find and fix the leak.