Montage:

This shelf can be mounted on rectangular tubes with a diameter of 30 mm x 18.25 mm. The distance between the tube and the wall behind it must be at least 32 mm, otherwise you will not be able to install the holders on the tube. I added a little tolerance on the models for 3D printing. I printed from PETG material. I also added a STEP file to the attachments for possible editing, who would need a slightly smaller or larger hole for the tube. I connected both parts with M6x 50mm stainless steel screws and on the other side M6 nuts.

Printing:

I'll leave the print settings up to each of you, but I printed with a layer height of 0.28mm, a 0.4 nozzle, 4 perimeters and a 12% infill and the strength is more than enough.