I have made a simple spoolholder for the ultimaker that can hold a spool with a diameter of 25mm to 57 mm. It replaces the original spoolholder at the back.

I was inspired by:
http://www.thingiverse.com/thing:106867

You will need:
4 ballbearings size:
diameter: 19mm
height: 6mm
Hole: 6mm

1 axe M6
3 Nuts M6

In the back of the bracket you can put 1 M6 nut. The other 2 are for the axe the hold the spoolholders
1 drilled a 6 mm hole in the original holder for better strenght.

You can cut the m6 axe at the length you desire.