I think if you could find an aircraft surplus 6" X 2 ft oxy tank it would make a great one ala old Cleveland M/C. On one of mine I used 4-1/2" exhaust tubing with bulged ends I made by putting 4 pcs 16 ga. on a 4" pipe collar and applying 6000 psig with a rubber stopper with a press. The tanks were mounted in straps on a saddle bent over the top tube. The caps are from auto power steering reservoirs. The shutoff valves were 1/8" Hoke needle valves. The tops were silver-soldered on top with 3/16"' brass rods threaded 10 x 32 with pointed ends extending to the outlets at the bottom. It was built the way they looked in the 50's It had a 3 hp. Clinton from a 1956 Yazoo high-wheel lawnmower.. It was on a Hopalong Cassidy Rollfast I had bought for $12 and too late discovered a restored bicycle like it for $1600. It had the Rollfast spring fork. A friend bought it for $250 for his car and bike collection. I am going to try to include a picture of my current one with the 6-1/2 and 3 sp. Sturmey-Archer tranny as a jackshaft, The tank is Hodaka Ace 90, I like it anyhow. You might get lucky and find a peanut chopper tank.