[Nasher]

In my head there is another option with an Ali tank, but I do need to put some more effort into working out if it will be significantly cheaper.

To my mind the major expense of an Ali tank is getting the complex curves in place, welding the panels together and finished to an acceptable standard. The use of an English wheel and proper time served Panel beating skills on Aluminium don't come cheap.

On the other hand, welding a multitude of flat Ali panels together to be fuel tight is skillful but quicker and a little easier to achieve.
My TIG welding is not up to doing the whole job, but I could cut the panels and possibly tack some of it together ready.
Then, I am very confident in taking a mold off a tank to produce a Fibreglass cover and finishing that to a high standard. A bit like more modern bikes.