I set out to design a bolt on filament guide. I quickly realized I could design a snap fit that would hold very well also, so I ended up designing one of those as well. I used the extra t nuts included with the Ender 3 and some m4 x 8mm bolts(one bolt is plenty strong, but there are openings on both sides).
Currently I have uploaded closed and open bolt on version and open snap on version. The closed version will hold a piece of tubing to aid glide, but is really not needed.
Make sure it is orientated on the bed perfectly flat, in Cura it took a bit of fiddling to get it flat.