UPDATED Feb 6, 2015: I changed the built in support material, added a recess that will fit an e3d-v6 better (the retainer for the ptfe tubing). I also made slight tweaks to the i3, 1.75, mk8 version's nut traps. I'll reupdate with i2,simple metal, mk7 and 3mm versions soon. Also, I made some changes to the fan shroud attachment, and the actual shroud itself that's working really well.
UPDATED Jan 9, 2015: I added my mount and shroud for a fan (intended to cool the plastic coming out of the nozzle, works with i3). I included the solidworks assembly that I used to position the fan optimally. It is adjustable and fully modifiable. I've added the .step files to make it more adjustable without solidworks.
I've also added an extended e3d 40mm fan shroud that I made when trying to use an e3d v5 on my i2 (for any extruder in general, but also with my i2, 3mm, MK8 extruder) and I couldn't fit the e3d hotend cooling fan under the i2 carriage. Easily modifiable to make it for a 30mm fan.
UPDATED Jan 6, 2015: I've added an experimental way to finely adjust the height of the extruder mount with m3 nuts and bolts. Beta for now, but I plan on testing them some time this month. I've also added MK7 versions of the extruder for i2 and i3 printers (or for any printer that secures the extruder with these standard hole locations) but they're untested for now as well.
UPDATED Nov 1, 2014: I've added better supports, (i2 mount works now) height adjustment for dual extrusion and a possible fan mount. Check them out.
I loved the extruder on my PB simple metal and want to use a similar one on my other printers. I also liked one of the mods that made it better at printing flexible filaments (thing 403438). I went ahead and redesigned it from scratch in solidworks, sorry no openscad for now, (maybe in the future, it should be simple) It has all three parts and uses a few different parts than the original.
The nut holder is a hex hole instead of just the groove (D=7+0.4mm diameter to fit a standard m4 nut, but you could change this to 7.7+0.4mm to fit the original nut. I used an m4 bolt and 624 bearing (4mm axle, 13mm diameter, 5mm thickness). I tweaked the arm to fit with these, but these are more common than the 625 bearing they use. The axle/pin is similar to what they use but may not be directly swappable. Everything is positioned for 1.75mm filament, but should be lined up so that you can just drill it out to 3-3.175.
I made variations for a mk7 (14mm cutout for 12.5mm diameter gear with 11.56mm hob diameter) and mk8 (10mm cutout for a 9mm diameter gear with a 6.9mm hob diameter). The MK8's smaller diameter necessitated moving a few features over by 2mm (11.56-6.9=4.66/2=2.33~2mm) I also put in a recess so a grub screw can stick out a little bit without interfering.
This can be attached to the PB SM with stock screws by inserting nuts into the nut traps. It can also be used on an i3 by printing the appropriate part and mounting as any other extruder. All the parts should be symmetrical (I think?) and you can set up dual extrusion by putting them side by side (just enough clearance. That's all I can think of for now, it works at the moment, but it is a work in progress.