Tight fit for M3D spools. Works great with the printable arm: http://www.thingiverse.com/thing:739342
I made this since I disliked the "Spool Spacer" (http://www.thingiverse.com/thing:753630) design as it was too fragile when printed with PLA. Wanted something a bit more robust that could work when printed in PLA.

Only need to print one as this will go all the way through spool.

My first upload, so hopefully someone finds it useful. :P

Made in SketchUp Make 2015 exported to STL.