Finished printing the new extruder and assembled it.  Still used the left-over stepper and tensioner parts I up-cycled from the M1.

Also modeled and printed a new rounded nut for the extruder tension bolt as the old one was tearing up my fingers.  

I left a single layer of extrusion for bridging where the bolt head ends.  It's simple to drill out by hand and results in a much cleaner print.

Much better on the fingers too.

Hardware source updated in Github.