The Grumpy Grease Monkey mechanical engineer.
The "professionals" just mig weld the patch panels in quick and dirty then cover the whole mess up with bondo.
To avoid that I did my own body work without any bondo when I restored my '36 Chevy PU in the early 1970s. There are probably many ways to weld in patch panels but here's what worked for me.
1. Made patch panels sized for butt welding.
2. Surrounded repair area with wet towels to absorb heat.
3. Tacked panels at approximately 1" intervals with Victor oxy-acetylene torch with #000 tip installed.
4. Welded sections approximately 1" long then cooled area with towels before welding the next section.
5. Hammered the welds to lower their profiles.
6. With hammer, dolly, pick hammer, adjustable Vixen file and "slapping" file worked the area to blend highs and lows for correct contour. With a thin rag under your palm you can actually feel irregularities that you can't see.
7. Shrank the "oil can" areas that inevitably result from the welding.
Metal shrinking is another whole topic that you may already have mastered.
I'm a geologist, not any kind of automotive professional. So I figure if I can do it, anyone can. Like anything, it just takes time.
The results were good enough for my restored '36 PU to win its class (T-2, 1929-36 trucks) at the 1976 VCCA National Meet in Colorado Springs competing against the mega dollar trailer queens. I throw that statement in to make it undeniable that a motivated amateur can do body repair and painting as well or better than the "professionals".
You can do it! Good luck.
get all the areas to be welded carefully cleaned ,
on both sides for at least an inch back on each side of the materials if you can do so,
with a rotary wire brush.
make damn sure you've removed ALL flammable materials and disconnected the battery cables before welding on a car.
have a decent fire extinguisher handy,
(preferably CO2 as it won,t make a huge mess like dry chemical will)
make an accurate replacement pattern panel from poster board,
to use before you start dealing with a sheet metal replacement patch panel
ideally use a metal forming tool to offset the panel edge so welds require far less grinding to leave a flat surface.
use a good anti-corrosion paint after welding the surfaces.
temporarily use clecos or pop rivets to hold panels in place for welding.
laying wet towels on adjacent areas, to absorb excess heat,
and welding in about 1" long sections,
then skipping to a new area, as distant as possible, on the panel,
to weld while that area cools,
tends to reduce panel warp issues
Provides a perfectly beveled 45° to achieve solid, full-penetration welds.