I think you would be better off to spend the money on the Brownells tank. I tried to find an acceptable substitute but ended up buying one because I could not find anything that would work well.
I tried something like spam4 suggested with galvanized roof gutter material and attached endcaps with high temp caulk, but it didn't work well. I kept having leaks and finally just gave up. The black iron pipe could work, but I don't know how you would split it lengthwise.
I did make a homemade pipe burner that works like a champ for rust bluing, however. I couldn't get the water to boil using a turkey fryer, so I had to improvise. Let me know if you want me to let you know what I did.