I found this design or one of its remixes a few days ago. I recently re-did my direct drive modification to my Ender 3. I've found that this works great on the top extrusion feeding the filament straight down to the extruder/hotend. I may try to remix an endcap for the spool holder, but I haven't had the spool fall off (from left to right movement on the X axis) yet. YET. lol
EDIT: I did have to do some testing and adjust my Z maximum height. But I don't print a lot of big stuff, so it's no big deal. It can be mounted on the back side of the top extrusion to avoid that, but it would probably need a guide to make it more efficient there. As it is, it doesn't need anything because it basically feeds straight into the extruder.