Kossel Mini top mounted spool tensioner magnetic

I made a kossel mini from think3dprint3d and over the moon with it.... apart from the top mounted spool holder runs way too smoothly on a bearing. so the spools have a tendency to unravel, which at times have caused a loop to fall under the spool, tighten and lock up, causing the extruder to stop and the print to fail.

This is a tensioner which places just enough tension on the spool to stop this but still allows the filament to run smoothly.
it also allows free movement but a constant tens