Good advice so far... When you get to the point of installing the new panel (trimmed, positioned, & fitting good), put a stitch weld about 1/4 to 3/8 inch long at the center of one side, then go directly across from it on the opposite side and put another stitch. Then do the same thing with the front and back edges; put a stitch weld in the center of each. Next, stitch each of the four corners. Then put a weld halfway between each of those, moving from side to side, front to rear. Keep putting stitch welds halfway between the previous welds, moving side to side and front to rear. This way, no area gets too much heat at any one time and warpage is avoided. Keep doing this until the insert is stitched all the way around. You can eventually end up with it welded solid if you want to go that far, but it's not necessary unless you are going to "metal finish" it and drive it around with no paint. When I do one, I usually stop when I get to the point where I have 1/4" welds separated by 1/2" spaces. Then I finish it off with Dyna glass or Tiger Hair waterproof filler.