I’ve never seen any patch panels for the bed, but I haven’t tried very hard to find them either. Here’s a thought. If your bed is rusting out in a few places you can see, it’s probably an issue in areas you can’t see as well. Have you considered just replacing the bed? There should be beds in good condition sitting in junk yards all over in the southern and southwestern states. I had to replace the bed on my truck years ago when a drunk driver ran into when it was parked. Painting the new bed to match was the hardest part of the job. The actual bed swap was easy. You’d be surprised how light they are.