Looking good!
You could use an old seatstay or chainstay for the brace.
I've got a ton of leftover steel bar and a few sissy bars, and frame parts too. Maybe I could integrate a rear rack.If you can bend the back and front edges of the metal sheet up, it will make that plate very much stiffer - the greater the depth you can bend up, the better.
No matter how thick the sheet is, it will bend when you sit on it.
I found a piece of steel online, I won't need to cut it, just drill holes.