Printrbot Simple Spool Holder (v2 Updated)
I made this because I was unsatisfied with the available choices for spool holders that could be made within the Simple's small build volume. It's a work in progress, but it gets the job done currently.
Edit: I have now changed out the STLs with a second version that should work better, and without any glue. Hopefully these work better with everyone--I'm pretty happy with how it's working so far. Instructions have been updated.
For reference, this was designed for a Push Plastic spool. It measures about 72mm in width.