UPDATE (25/07/09) : Small update for the cylinder part. I also printed new one myself to check if everything fits ok. Reinforcement bar works great, no more flex in the middle part of the spool holder. If the fitment is to loose you can apply some glue when installing reinforcement bar. 
UPDATE (25/06/30) : Added some extra fillets and chamfers. Shortened the legs, so 4 screw holes are left instead of 6. Shortened the model itself by 10mm. Added reinforcement bar for extra stiffness.   

Needs minimal assembly. 

1xM4 - 4mm heat insert
1xM4 - 12 screw
3x608ZZ bearings

I used normal auto supports for the main part. (paint red for screw holes)
No supports for other parts.
Snug fit for the cylinder part so nothing rattles.
3-4 walls.
0.24 layers.
20% infill.

If you are familiar with modifiers, extra infill and an extra wall on the bearing housing piece wouldn't hurt. (see last photo)

Attaches to the shelf with screws.
Can use scarf joint seam for the cylinder.
This spool holder is not designed to be used on the table.
If there's any problems with cylinder tolerances you can create your own, easy to do. Current cylinder has inner diameter of 34.8mm.
If the cylinder fits, but doesn't spin smoothly, you need to sand down the Z-seam inside the cylinder or iron it out with soldering iron with a roundish tip.