UPDATE Feb 2013: Well it doesn't print well with my 0.5mm nozzle. The wall where the hole is close to the side is too thin, see photos. I may try it scalled to a larger bolt size and play with slic3r settings a bit.

This is not a direct derivative of makerslide http://www.thingiverse.com/thing:8992 as it does not include the spacers.

This is based on the spec at http://www.buildlog.net/sm_laser/drawings/b18001_rev_3.pdf

I couldn't see why a metal one would be any better than a printed one. So I made one in SCAD.

I've also included a plain spacer to match.

It's also all zipped up for easy download.

The original STL of the Eccentric Spacer is at http://www.makerslide.com/

It and my version is for 0.281" 9/32" 7.1mm hole with a M5 bolt.

However you can modify easily for other sizes.

Print
Shove in a 9/32" hole
Mount a bearing with a 5mm ID, using a M5 nut/bolt
Adjust the bearing to fit the rail mount using the hex section of the spacer.
Use the plain spacer to have idler wheels at the same height.