Idea for Tank
Why not use PVC?
I believe one can make a filler opening and epoxy in place, perhaps even the top of older tank with cap or MSR bottle...
Outside of that, PVC, it seems to me, is suitable material, will seal fine, you can customize it to your bike frame specs, like length, etc. easy to do and fasten to bike for quick removal as well, using some sort of "flip-locs."
Am thinking of using PVC in combo with high-temp silicon, as small expansion chamber for short muffler for a T-3300.