Update 31 Aug 2014: A "tighter" fitting version has been added. This may reduce wobble/enhance precision slightly.
Update: Looking for the E3D v6 mount? It's here:
This mount is designed to hold an E3D v5 hot-end. In addition to holding the hot-end firmly, it supports the rods, maintaining a 90 degree angle between them thereby improving positional accuracy.
3D printing with ABS creates a mount that works well. This model is also suitable for CNC, which should create a much more rigid structure.
Aside from the E3D, other "Groovemount" hot ends should also fit.
While the piece has screw-holes for maximum tension, I've found zip-ties are an adequate and convenient way to keep appropriate tension between the pieces.
CAD by Davide using FreeCAD
Print the lower piece as is, and the upper piece upside down.
File and lubricate the pieces until they ride easily on the rails.
Insert the hotend, and keep the pieces aligned with cable-ties, toothpicks or screws.