This thing has been updated 25/6/12, use upper section 1.3. The 1.2 files are missing because I forgot to update while experimenting before moving out of my last flat (sorry), but 1.3 is now much easier to get wide-PEEK-block style extruders through fully assembled since I ruthlessly cut away unnecessary plastic.
I designed this so that I could more easily use a Prusa-style belt setup while using the original 624 bearings on my Sells Mendel x-carriage.
PLA bushings just seem to have too much friction to me, and linear bearings can't be put on or taken off the X-axis without removing the smooth bars from the end brackets. (Not to mention the extra expense just to try that out).
By making the space for a tool head or hot end to fit through rounder, instead of the old long-narrow shape (on left for comparison), this allows you to mount a tool at either 90 degree angle, like the Prusa x-carriage.
I also put holes on the side to mount a 40mm or 32mm PC north-bridge fan to cool any plastic extruder filament drive.
The bottom part has now been updated so there should be enough room to fit a standard mendel hot-end in there at the 30 degrees angle that greg's hinged extruder mounts it. In my case I simply took the original lower section of my mendel x-carriage, snapped off the part that used to support belt clamp bolts, and cut away some material from the bottom.
My models were quickly based on Idaholion's solidworks models of Mendel parts (picture of original for comparison), so I had to rotate the STL files in blender after exporting them (you will find the STEP files are at a silly 90deg orientation.
I also noticed when I went to upload the updated files that I had previously posted the wrong .blend file from a different thing, derp. I can't believe the 16 people who downloaded it by then didn't tell me though :P
Assemble as per the instructions for the original Sells Mendel X-Axis: http://reprap.org/wiki/Mendel_X-axis
...with the exception of the belt clamps, which no longer protrude bolts into your hot-end space. For these you can use standard Prusa clamps on the protruding side of the top, or use the included adjustable belt clamps by facing captive nuts inwards then swinging your belt-ends over the top and back under each clamp, to tighten them up with an M3 bolt after fixing the ends in place.
Also you may need to use more appropriately-sized M4 bolts than those ones facing diagonally upwards in the original design (I already do), so that they don't collide with the top part.