Simple filament guard

Simple filament guard for Prusa i3 family printers
If like me, you mistakenly bought 3mm filament instead of 1.75 or have some aside this is a very simple device to prevent the regular filament flow out of the reel.
Brittle filaments like PLA are not a good choice for the loop.

If the reel arms of your printer can slide along the top bar, you don’t need to remove the device while modifying their positions.