Shelf edge thingy

My girlfriend has lots of paper rolls in her office. She just wanted to have some shelf bracket without boards on, but she needed something to ensure that the paper rolls did not roll over the edge.

Therefore these edge thingies. They are designed for millarco shelf brackets (I have seen them several other names though). I have no idea if something similar exists somewhere outside Denmark, but else this design might just serve as inspiration.

The "thingies" are designed for 3.5mm screws.

(This style shelf brackets: http://www.xl-byg.dk/produkter/bolig-fritid/hyldeknaegte/rack-system/millarco-pulverlakeret-track-system-rack-system-hvid-17cm.htm)

Update 20170115: Added a version with 20mm high knob. The original design is 35mm high.