Figured out that most of the print failures I was encountering could be traced to heat creep (premature softening of filament as it passed through the extruder, causing the drive to slip and/or jam). Active cooling to the rescue.
Parametric model available as a public document on Onshape. I'm not sure how to link to it directly, but you can find it by searching for
40mm fan adapter plate for 2013 Printrbot Simple extruder. The document contains two part studios: one for the extruder plate plus one for my DC power connector mount.
Started with Joel Crane's 30mm fan mount plate. Initially hacked with OpenSCAD to add a larger diameter opening and assumed the 40mm fan bolt pattern would just match the extruder mounting bolts. Not quite.
Imported to Onshape using FreeCAD to convert the STL to STEP format. Used the imported object as a reference for a fresh sketch. This version (posted here as
jmd-extruder-plate.stl) adds a spacer to clear the extruder mounting bolts and a new set of offset mounting holes (45mm diameter) for the fan.
Built with a 12v 40mm fan and DC power jack from Radioshack (!). The jack connects to a compatible power supply I happened to have for a small rotary multi tool. Evidently it would be possible to power this from the heated bed circuit on the printrboard, but for now the separate power supply is simple enough.