Some fire extinguishers are made of aluminum and are already tapered on one end.
I got mine free from a fire extinguisher service.
They test them to something like 260 PSI, so they can fail the test yet be a sound tank for unpressurized use.
I found one that would fit in the back of my J along with the existing 10 gal tank. It holds just under 3 gal.
When I got it, it weighed 7.5 lbs I've got it down to 5.2 lbs by using a lathe to thin the top and bottom walls. I'm in the process of tumbling it to smooth and thin the outer walls.
Still haven't decided whether I will put it in. I really like the way it flies with just 2 gallons of gas.
If I can figure out an easy in/out mounting and plumbing I'll put it in for sure.