RJ, you can use prety much anything.
I've used 35kg gas bottles in my first and second build, about a foot in diameter, and about 4' high. I'm reusing thoses adapting my second build to run on bigger pipe. I've used a 500L watter tank for my batch rocket. I'm trying to get a rectangular box made, 20"x20" and 4' or 5' tall, so i can pile up bricks around, and have good contact. In the us, you could easily get air con square tubes. We've considered
concrete suage pipes, well elements. Pozzolan chimney elements. Metallic lockers with the door and vents welded. Etc!