That's a thirty dollar question...

The new tanks should be made with the same taper as the hull. The easiest way to do this is to make a template out of cardboard to the shape you desire for the end plates, then measure from the inside edge of the tank every few inches until you reach where the tank will be.
Also make sure the outside edge of the hull is straight, if not, more measuring, or make the tank straight and allow for a void.
Once all your measurements are made, they can enter that into CAD for the cutter.
Looking great, you'll be glad you did this.