Summary

The two previous spool holders that I printed didn't quite work for me (I'm sure it was as printing error on my side, not a design error on their side), so I decided to combine them into something that did. It still catches every once in a while, so for now I've put a layer of cardboard (with the center cut out) between the bottom of the spool and the top of the bot. That seems to do the trick.

Also, I wrote about the embarrassingly large number of prototypes it took to design this here: http://www.publicknowledge.org/blog/3d-printing-non-engineers-rapidly-prototyping