This gave me an idea. Have you considered buying a used higher quality rack and modifying it? Might save some time and get you what you want with less new material, and sort out some of the harder parts to fabricate because they’d be done already, possibly only requiring a little bending to align with your eyelets. Plus you’d be saving something from someone else’s scrap pile.

Around me there are a few community run used bike part shops. They typically have lots of racks and other parts kindof organized into piles or boxes by type. I can usually find something close to what I need if it’s not too specific.