I started with the modified paste extruder available on thingiverse, but quickly found I needed a more accurate extruder. I moved to a modified syringe pump based extruder but needed one that worked how I wanted (and could do dual extrusion! I took a (relatively) cheap linear actuator I found on ebay (http://www.ebay.com/itm/STEPPER-MOTOR-LEADSCREW-LA23GCKB-1-LINEAR-ACTUATOR-/290996217983?pt=LH_DefaultDomain_0&hash=item43c0b9e87f) and designed a small "box" to constrain it. Briefly, I placed the motor up top and syringe at the bottom. The linear actuator stepper progresses a threaded rod (1/4" acme 16 threads per inch dual lead - 8 threads per inch per lead, effective pitch) with a flag (in a groove) to keep the rod from rotating rather than progressing. There are two sets of mounting holes (30mm apart, 5mm from the edge to give clearance between up to two extruders), symmetrical flag grooves, and holes for access to tighten the flag bolt - all so you can print two of them and mount them, side by side. (Pictures soon).