Just a simple spacer for the spools from http://supply3dpla.com , as they ship december 2011.
I'm starting to run out of 608 bearings, so I tried to just use a 8mm hole in the middle and thread it directly onto the rod, and it works surprisingly well.
Still, I have only used it for a week, so I don't know how it holds up in the long run.
Quickly put in while it's hot off the printer. The spool sides is pretty thin and weak, so don't apply too much pressure or it'll crack.
Drill out the centre to 8mm to make it a good fit, we want to make sure the spool rotates around the centre so as not to risk the spacer wearing down and falling out.
NB: As it rotates it slowly eats it way along the threads, so make sure to keep an eye on it. I use some tape around the smooth rod, and it keeps it pretty much in place so far.
The very simple OpenSCAD file is included if you want to tweak it. It's just a quick hack though, and thus is not pretty at all.