Filament Spring Clip

*EDIT: Please check my improved design: https://www.thingiverse.com/thing:3023778

Old stuff:

For 1.75mm filament

I printed it with PLA and the tension is just fine for me

-Slightly open the gap with your nail
-Pass the filament end through the complete hole
-Feed the clip along the filament until it meets the filament end again.
-Clip the filament end into the side-gaped hole.(a click sound might be heard)
-Pull the filament to tighten the spool.

Tolerance is narrow so an accurate printer is needed or you can size it up slightly in your slicer.