These go with Printbit's excellent Guitar Understring Radius Gauges (here: http://www.thingiverse.com/thing:156298 ). I printed them but they needed something to hold them. So I made one.
This piece has sockets for all 9 gauges plus one spare slot - they clip right in with a little push, hold tight, and pop right back out when you need them. And they're arranged so you can see the sizes all at once!
Print with no support (the gaps are small). Infill %, speed, etc doesn't matter so go nuts. Mine was at 25%, 50mm/sec.