Spool Holder - keeper nuts

If you are using a threaded rod (3/8 inch) as a spool holder these keeper nuts will thread onto the rod and hold the spool bobbins (such as [thing 316826](http://www.thingiverse.com/thing:316826)) in place.
rev_A - too tight, hole closed slightly when printed.
rev_B - fixes the hole size to account for shrink
rev_C - removes the sharp points that hurt my fingers