Update 7/24/2018: Someone asked for a matching right-side endcap so I've added one.
I couldn't get another Ender filament guide to fit on my printer as it required pretty tight tolerances so I made one that just snaps onto the top of the 2020 extrusion.
Slide this on from the end of the extrusion
Snap it on from the top about halfway, then slide it on the rest of the way.
You won't be able to snap it on from the top in its final position unless I add a slice where the clip and arm meet to allow the clips to flex. This might weaken the part or make it looser than I wanted it to be so I didn't do it.