This is my design of filament guide. I have my drybox spool holder place under my printer, so I needed the filament guide with which I'd be able to pull the filament from below the printer. I came up with this design. It is simple yet robust, it has served me many hours of printing now, and had not failed once in any way. It is designed to accommodate a ball bearing with outer diameter 26mm inner 9mm and width of 7mm (that was something I had laying around). I think it's pretty neat, as it feeds the filament on a straight line to the extruder, and it also is jamming, scratching free because of the bearing.
There is one flaw in the design however. the "locking key" is wider than the bearing inner race diameter, but I have realised it too late. However, simply cutting extra plastic with clippers solved the issue. That's why I've marked it as "work in progress"
Comment if you think that I should fix the model, or make another for a different bearing design.