Fitted on this year by hanging it alongside the hull under the cockpit locker floor. Cirtainly the job was much easier than fitting a new rigid tank. Having a wooden hull I was concerned about condensation under the tank so fitted slats to the frames for the tank to rest on - improtant to stop abraision. I used nice soft spruce. The only problem has been that the unions wept a little, so put them where you can get at to tighten. I didn't and am waiting till I have used up the fuel to tighten the bottem one! I think lack of water in fuel is a significant plus - as there is now air above the fuel there is no wet air so no condensation. Interesting to see if this is the case over the first winter.
If yours is a petrol engined boat then I know of no manufacturer that makes what you want. They are I believe all designed for diesel only, don't be tempted to use for petrol, it could prove to be extremely dangerous!
Have you considered a rigid plastic fuel tank? No personal experience, but they can be made to fit almost any available space. See adverts in mags.
I have an inflatable water tank from Plastimo. This is my second and I expect at least six years life. Abrasion seems to be a problem unless very carefully thought out.