Here is a nice looking filament spool holder to go with my Kossel Mini 3D printer. It uses printed parts and 15x15mm Mitsumi extruded aluminum. The rod is 8mm or 5/16 diameter. You can make this holder as large as you want.
The length of aluminum I used is:
You can download the thing here: http://www.thingiverse.com/thing:259003