I designed this filament filter guide for the Folger Tech 2020 Prusa i3. It serves as a filament guide, and a filament filter all in one.
To use the filter, put a piece of foam sponge (or whatever else you like to use for your filter material) around the filament, then slide the filament through the side of the guide and the filter down into the square pocket. Slide on the front cap to keep the filter from getting pushed out of the top, then drop the retainer ring over the whole thing to insure the cap can't fall off.
Alternatively, you can slide the filament into place in the guide, and stuff the filter in around it before putting on the cap and ring.
Some people put 3-n-1 oil or canola oil on the filter, and say that helps cut down on jams.
This is probably overly complex, but it started as just a filament guide, then I added the filter box, then the cap, because the filter would occasionally get pushed out on a retract, or when the head moved from the extreme left or right back to center under the filter. I added the ring after the cap fell off once. I could have made the cap tighter, but I wanted it to all be easy to quickly slide together, and the ring is really easy to use. All together, everything is held in place and can't come apart by itself, regardless of how much the printer or the filament moves.
I use this with my new spool holder which you can also see in the photos.
Folger Tech 2020 Prusa i3
I use 0.2 layer height, and 30% infill, but it probably doesn't matter. Use a T-nut to hold it in place on the 2020 bar above the printer.
Ignore the rough print in the photos. I revised the part to use less fillet on the cap, and it will print much smoother than the one in the photos.