This bracket is designed to make use of the dead space under a shelf. These are the generic shelf brackets from Lowe's, holding up a 12" deep shelf. 

The rod is a 3/16" aluminum round bar, available at Home Depot or Lowe's. 

The brackets are printed on the U-shape face, with light supports for the round boss. This ensures that the opening slides easily onto the shelf bracket. The rod opening is slightly oversize to account for any drooping in the print.

Each bracket has a 4mm hole on the back for use with M3x4x4.5 heat-set inserts. I originally planned to use that as a set-screw to hold the brackets in place on the supports. Instead, I cut the aluminum rod just a bit long so that the rod is acting as a friction fit for the brackets. If the bracket moves at all, I can add the inserts and set screws at a later date.