Simple filament sample spool that uses 10mm x 3mm  magnets for closure.  I found that many of the sample spools I saw used too much plastic and/or the center area was too wide for some of the tighter wrapped samples.

Notes:

1. I suggest only using 2 10mm x 3mm magnets for each side. As you can see in the pictures, I only use 2 of the magnet holes and it holds really well. I actually have twist the sides to get them apart. I think 4 would be hard to separate.
2. Be very careful with the magnets around children!!
3. Make sure to orient the magnets polarity and triple check before pressing the second magnet in!
4. While they start as press fit, the magnets will come loose over time so I suggest using a couple drops of CA glue once polarity has been verified.