Pretty self-explanatory. Slips onto the front edge of a "1-by" (actually 3/4") shelf. There's a counterbored hole for an optional #6 screw on top to keep it from coming off accidentally. Print as many as you need to support the length(s) of stock you want to hold. The wider ones in the photo were a prelim version before I narrowed it down to 20mm to save printing time.
Customize it here on Tinkercad if you like: https://tinkercad.com/things/k4q8wVmi8wr-rod-stock-holder
Print it laying on its side so that the layers give strength against the load put on it. I used ABS, 0.25mm layers, medium-light infill.