Modular spool for filament samples

NOTE August 2020: I've created a new modular reel thing, with a similar mechanic, but different enough that it doesn't seem to be a new version of this design. So if you like please see my 'modular sample spool 2.0'.

So, I joined Makerbox, and was looking for something to hold the samples. I looked for others' solutions tot his, but all fell short for me for one reason or another--spool hole too big or too narrow, too wide, too narrow, not deep enough, blah blah blah. I'm finicky about things like that.

There are two parts, the main filament-holding 'reel' and the 'end-cap'. You'll need at least one of each for this to work. I usually work with two reels and the one end-cap.

The spool-part that holds the filament has a diameter of about 10cm. The inside part that sits in the spool holder (on my machine) is about 4 cm. There are four 'tabs' on the inner spool that fit into the four 'slots' in the endcap (or another reel). My spool holder holds 3 reels and an endcap comf