Well, I can say that aluminum is one of the easiest materials to work with, and it's relatively easy to aquire, and cheap, and light. Very light. I use vulcan g-51 braze rods for aluminum. It's a breeze to join nearly all aluminum parts together with no effort.
What I'm getting at is if I wanted to make my own, I'd go that route. On another note, I'm currently building a round barrel shaped tank from a 5L hieneken beer dispenser. I thought it was aluminum, but it's actually steel. I have some seal-all, a gas and oil resistant adhesive, and an old tank, and some closed-end rivets. I'll chop shop it together, spray paint it black, and strap it on baby. Ahem, yeah Uh... Well I'm going to make it work, can't do welding effectively, the vibration would tear the tank apart if I welded, so carefully placed adhesive rivets will do.
I had a suzuki ts125 gas tank on it, but it leaked too much and I'm not sure I want to seal it or anything, I might just pull the gas cap and fill tube area and use that as my cap area (it's a flip open cap, can't be taken off and lost, and can be locked if parked in a questionable area.) also would pull the petcock area too, as it's a brand new petcock, and it's got mounting holes already made for bolts.