I think you could get close to 48" with a 12" reaction chamber. Ifyou went about 36" - 40" with the 12", then necked up another 10" - 12", I think it would work just perfect. With the 18" diameter I would say 24" max height on the actual 18" part, thena nother 6" - 8" on the neck. That might be a little generous on the 18" diameter sizing too.