Took a random spool cover I've had for years and made a quick external version for my Qidi Q1 Pro. This is specific to the Q1 Pro's rear spool holder, there are holes in the design to allow for mounting it to the arm. I used a small screw; this is necessary for the filament slot to stay in place. The pictures show the factory mount from Qidi, although I'm sure you can adapt this to any others out there using the holes in the model.

I made this to be a very thin and fast print, I printed in ABS as its all I had on hand, but anything will do really.

You'll need to print the thicker spool holder for this to work as the inner diameter is set for that: Qidi Thicker Filament Holder by Michael Hathaway | Download free STL model | Printables.com