I am in the process of building a PRUSA i3 Mk2 clone using 3030 aluminum extrusion from 8020.net and thought it would be fun to make my own slot covers. I downloaded the CAD drawing of the profile and imported it into CorelDraw and drew an outline profile of the slot cover and imported it into Tinkercad. It fit perfectly the first time so now I can't wait to print more when the printer build is complete. If you are interested in building your own printer check out https://www.thingiverse.com/thing:1692666. It just snaps into place.
If you want longer sections of this thing, import it into Tinkercad and stretch it to the length you want.
Also, check out my 3030 Cable Tie Down posts that snap in place. Easy to slide into position. https://www.thingiverse.com/thing:2843530
Added FINAL. Fits a little looser.