I built a box around a dozen 24" tubes using the specs in the Sullivan and Weese book. It easily accomodates my 16x20 frame and is (relatively) portable. (I keep it in a closet, away from the curious hands of my three year old.)

I would agree with Sandy that building a printer around 48" tubes is probably at least as cost effective as the aformentioned 24" version, but space is going to be the determining factor.