Just use a regular nema17 motor and Bob's your uncle.
Two things have been changed from regular Greg's, one is the motor location, that has been translated, and the whole thing has been elevated over the base so elements protruding to the back (towards the x-carriage) are now above it.
Those of you using Budas hotend may want to use the new version with 10mm spacer included so it won't bump into the x-carriage.
It fits the standard x-carriage of Prusa i3 (30 mm version, which is the latest).
Based on dviejo's mod.
Source code is available, you can adapt it to any of the supported hotends. Please note a couple of files are included, now I have added them to the thing. Alternatively, copy the extruder scad to the Prusa i3 box_frame folder and the files will be found.
A pair of M3 bolts (30mm or longer) will be needed to fix the extruder to the x-carriage.
Video of an early version: http://youtu.be/E7ju4vxNtpI
I am using a pair of 50mm hex bolts, with springs and M4 nuts for providing the idler pressure.
I am told (thanks obijuan) that for 48mm body-long steppers you want to add 5mm to base_extra_depth value on the source to get additional clearance.
If you need extra clearance but you already printed the extruder, or if your x-carriage is 24mm instead of 30mm, you can use this part to fix any these problems: http://www.thingiverse.com/thing:81539