Snaps directly onto the shelf pins. Holds two 20mm beams. I used pine, but recommend something stiffer if you go for the wider shelf. The pine holds up for quite a while now but I've switched to 20x2mm steel nonetheless

There are only variants for the 30cm version. 

I'm attaching the CAD file so you can edit it, if you need a different diameter or the 50cm version of the shelves

Update

I've added a pin that you can print and use to mount the beams on the shelf if you got no original pins left. Printed them in PLA and they hold the weight just fine 👌

Print Settings

4 perimeters
40% infill (grid)
5 bottom / 5 top layers
no supports
brim for the pins