Summary

This mount can be fastened on one side of the Extruder.

Instructions

If the current M4 screw is not long enough you have to replace it.
The upper hole for the M3 screw to clamp is diameter 2.6 mm. You can cut a tapped hole or screw the screw in directly. So you don't need a nut.
For printing the slot I added a support on the outside which must be cutted of afterwards.
The mount is destined for a 60 mm LED ring but you can also use a bigger one.

Edit 23.2.2013:
I added the stl-file: LED Mount for Printrbot_mod) because I noticed that the hole for the M4 screw, to fasten the mount to the extruder, was to small (3.6mm).
Now I modified it and made the hole 4.4mm. Sorry.

Edit 25.2.2013:
I added the file: "LED Ring Mount for Pb.FCStd".
This is created with FreeCAD, a free software which you can download here: http://sourceforge.net/projects/free-cad/files/

Now you can change the height (besides all the other dimensions) as you need. All you have to do is:

Start FreeCAD
On "Start a new project" take "Part Design"
Open the file "LED Ring Mount for Pb.FCStd"
Click the triangle next to "Pad"
Double-click the sketch "profile" that appears.
Double-click the vertical measurement and change it as you need.

LED Ring Mount for Printrbot