Hi griffedi,
While I'm not familiar with the patch panels those vendors are currently selling, I can tell you this is a good way to make the repair, so long as the seam can be well hidden (depending upon how much originality concerns you).
I would expect the vendors to sell proper gauge steel, which might be a little hard to find locally to make your own patch panels. Even if not already flanged, that's a minor consideration and any trimming would remove the flange, anyway.
Flanging tools are easily found from a variety of sources. There are bench-mount and handheld type. Also, Eastwood Co., Northern Tool and Harbor Freight all offer air tools that both flanges and punches holes for plug welds (or for glue/rivets, if that's the way the repair is being done, although it wouldn't be my personal choice).
Assuming you get some sort of flanging tool, whatever one you choose, be sure to confirm it can handle the 18 gauge steel TR bodies are made from. Most modern cars use lighter 20 to 24 gauge steel and some tools just aren't up to working with the heavier gauge stuff.
Hope this helps!
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