We used the interior wall skins to seal some factory installed convection leaks, in front of the stairwell. Drafty stairwell, no more.
I also used the wall steel
to replace the plastic hump between the rear seats.
Shaping the wall steel, I was able to use simple hand tools:
standard tin snips,
ball peen hammer (a must),
diy brake of C-clamps w/angle iron...
all on the picnic table.
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The heavier, 16 guage roof patches are more material to work with. Takes longer to cut & drill, power tools are a must, probably specialty (metal shop) tools if you want a fine edge, complex shapes or clean brakes.
Even where the factory uses thinner gauge, it can oil can, 'volcano' or gather around their screws.
A thicker guage can help pull it all flat.
If there is an intended load, like a rooftop a/c, I'll want the additional thickness to maintain shape.

If you need to hold a specific shape or a load, use thicker steel.
If you feel that you just need to seal a hole but not hold a perfect shape, maybe the quick & handi-steel.
All of the 16g steel patches used on my rig (above) were given to me by EastCoastCB. They were pre-cut to exact spec using a plasma cutter in pro's hand.
If you're willing to do it the hard way, use 16g, predrill everything and obsess over any detail that might affect an air/liquitight seal.