[dsraven]

if sticking with the same size hole as currently shown, i would cut a strip that fits well, using your panel clamps. tack it in place, staggering the tacks around the edge so as not to allow the heat from the tacks to warp or pull the metal and also hammer on dolly each tack whie still hot. keep making tacks until they are starting to get close enogh together so a short weld can be made to connect the tacks, then do short welds with the same mindset as the tasks. weld, hammer on dolly, allow to cool, move to the other end of the patch, weld, hammer on dolly, allow to cool, etc. some guys will use compressed air to cool and, I have done that myself, but I always try to allow the panel to absorb some of the heat first and then I cool the whole panel from a distance with the air rather than do a close up blast on the hot spot. that is because mig welds are usually harder than a tig or a gas weld would be and work harden quickly which can cause cracking later. hammer on dolly when the weld is hot expands the weld area so when itt cools it is roughly the right size again. it is a hard thing to not get overzealous and not allow the piece to cool enough before welding again. martinsr has a good analogy for that, do something else at the same time to keep you busy and then don't weld again until it is cool enough to stick your tongue on. I have a few pieces of flattened copper pipe that I use as heat sinks. magnets also work but if too close they disrupt the electrical current from your welder. tacking a strip on the back side may help for heat absorption but I would remove it when done so as not to be a moisture trap. actually, a strip on the back side connecting fore to aft may be a good idea so the panel doesn try to get shorter on you when welding. I would just stress to do a few tacks on the front patch, a few on the rear patch, then walk away and do sometthing else for a bit until the panel is all the same temp.