I had some 8 mm carbon rods laying around and figured my 3D printer needed an upgrade for the spool holder. The rods are nice and long (50 cm each) and I originally intended them for a quad copter build, but I found rc-helicopter tail-rotors made of aluminum to be a better fit for that project.
The whole printer/spool holder assembly is quite high, but it saves space on the table at least. It's this high by design since the y-axis moves back and forth and having a bit of distance between the print head and the spool is good in that case.
Closeup of the spool holder.
For the center rod I used a threaded rod. This makes the centering of the bearings easier and also adds to the stability by allowing us to tighten the hold around the bracket from the vertical rod a bit.
Oh, and it rolls so much better than my last try. The design can be downloaded from Thingiverse.