40mm extruder fan adapter plate for late 2013 Printrbot Simple

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](https://www.onshape.com/). 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: