I have my rep rack shelves mounted up to the end of a wall, but i found that I had a few extra inches of EMT tubing overhanging that I wanted to utilize. I Found neon.blue's edge shelves and these were good but I wanted to be able mount the shelf securely to the tubes and the Rep Rack Hangers. So I added an internal press fit for the EMT tube to (snugly) slide into. I also added some screw holes so that the shelf can be screwed to the hangers from the hanger side. Lastly, my original intent for these shelves were to hold an extra spool in a pinch and so i added raised edges on 3 of the 4 sides so that filament can be set on the shelf without fear of it rolling off.  These shelves should be just wide enough to hold most standard 1kg spools. 

I printed these in petg and pla. I used 3-4 walls and cubic infill. Oriented with the screw holes facing up. I don't think the strength of the material is of too much concern since the tubes running through the shelf would bear any load on the shelf.