This is a directive of Makerbot's Horizontal Filement Spool. http://www.thingiverse.com/thing:3116. It takes about 3 hours to print, and the size can be adjusted to any footprint depending on how long you make the rod. It is designed to 8mm Metric, but my build is using 5/16, which works perfectly well, so no need to have two versions.

You can see what it looks like in this video:
http://il.youtube.com/watch?v=N4j2pJzM13Q

The HeeksCAD file is the raw solids before I chaffered (Some of the blocks might be a little off, it's before I finished it up for upload), so all can be modified completely. There is no Tube feeder included in that file because it's so easy to throw together (2 8mm cylinders, 1 12x12x22 block, and a 2 booleans, 2 minutes work in Heekscad)

(M8 can be substituted with no ill effects)

3 yards 5/16 rod
1 4\" electrical PVC pipe coupling (center of spool)
2 round disks of plastic, the harder the better, no need to the top to be nearly as big as the bottom.
3 foot 1/4 inch OD tubing
2 1/4 inch wing nuts
2 5/16 washers
64+- 5/16 nuts
Epoxy for spool (remember don't epoxy the top, you need it to come off, don't ask why I have to say this)
5 608 skate board bearings

My unit is ruffly the same size as the Makerbot Filiment spool, and I cut the Rod in the following pattern

6 315mm (outside and 2 bottom support rods
4 115mm (4 bearing rods)
1 135mm (spool rod)

You will need to run a drill though the tube where the wing nuts grab it so it doesn't add friction.