A bearing hub for my 1Kg spool from ultimachine.
STL spool ID = 51.87mm, SCAD provided to change as needed.
Uses 608 bearing by default
I'm using it with http://www.thingiverse.com/thing:22759 as a bracket
Update: Changed scad to be clearer on variable names. Added 1 layer skin under hole.
Make two of these, add two bearings, place on M8 rod on either side of your spool.
By default, the hole is closed by a .25mm layer to provide bridge support. Drill it out or set it to 0mm in the scad if you want to just make an uglier unsupported hole/bridge. That's actually how I printed mine, and it worked ok.
If the inside of the bearing hole is tight you can run a 7/8ths inch (or 22-23 mm, if you have such a thing) spade drill bit into the hole to ream the edges.