This is a crude design made using OpenScad, based on the other Spool Bushings out there.

This one will fit onto a 1.75 inch spool, like the one I got from New Image Plastics (their 10 lb spool). The skate bearings fit nicely onto the MakerBot threaded rod (or equivalent) and provide alot less friction so don\'t have to worry too much about possible feed issues. This is related to my other changed thing (http://www.thingiverse.com/thing:3760). The hole is not exactly round, so I made it a little oversized so didn't have to trim too much off.

Do have to worry about too much unraveling as the spools are generally not balanced well. To fix this problem my spool holder tilts slightly to one side, so I moved the spool so it would slightly rub up against the side, as can be seen in the pictures.

Optional: If need to secure the bearing inside the bushing, I would recommend using a hot glue gun (I didn't have to)
Have included the OpenScad file so change to your hearts content.

Print two parts
Slide the two bearings into the center of the printed part (might have to be careful when cleaning out the middle).
Mount spool on a MakerBot threaded rod (or equivalent) and start printing away with filament ease.