I tried for years to find replacement brackets for my closet maid system but they stopped making this particular part. They sandwich together to hold a straight section with a corner section to a support bar on the back. I printed them thick with a lot of infill and they came out really strong. I printed 8 of them and they’ve been great for over a year!
I had to create a couple of versions to work around differences in the distance between the top wires where mine were mounted. I'd recommend printing a couple draft quality prints to test the fit before printing them with the full strength needed.
Make sure to print this thick with lots of infill. I used 0.1mm resolution with 1.2mm top/bottom and walls. I also used 50% cubic infill. It's like a brick. Orient it with the flat part down with a little bit of support for the screw hole.