I've seen several filament guides on here, but almost all of them require removing the filament and running it through a loop, or seem like the filament would come out of it easily. I decided to design one that you can install without having to remove the filament or deal with any of that mess. All you need is an M4x8 bolt and a T-Slot Nut to install.
I have printed this already and it came out looking great, but the version I printed had an upper slot guide that got a little wonky, so I modified it to hopefully print easier. I have not yet tried this new design, but I'm sure it will be fine.
Hope this is useful to you!