They sell backing plates with handles. Eastwood has one with built in magnets, that's cool. I use copper pipe flattened. They are long and can be bent and rebent to fish up into place and held with one hand or a magnet or a stick or a jack. They are kind of "heat sinks", but more importantly they just allow you to keep welding until the gap or hole is closed, as a temporary backing plate. The weld builds instead of burning the edge away (burn thru).
Your link shows a fellow making a "heat sink". Which he says dissipates the heat. Probably not much. He has it "pressed" up against the area. Futile effort not to be used as a guide. A heat sink must have many fins and be in tight intimate contact and be thick. This brings up a very important point I forgot. You want to ONLY weld stacked spots. (spot stitch). One spot here and one spot there. Far apart. Then start to weld onto about 1/3 of a spot to create a continuous weld. Do not bead weld anything thin. If you see a guy doing that on YouTube, ignore it. The key to remember is to wait. Even if the next weld is far away. Wait until the metal is cool enough to touch. This can be seen as over kill. But you only have one cab and one patch at a time so why would you even dare take a chance on warping. Once it is warped, you are done, you are in deep trouble. Do not get in a hurry. It will get done.
Grinding can cause much heat. Not nearly as much as welding but you can warp with grinding. This is where an air gun can be used to cool the grind so as to continue. Grind with a die grinder and switch to sanding when getting near parent material. It is very easy to thin out the material by over grinding. So be careful. It's all very satisfying and rewarding therapy.
Remember, 50% of Mechanic'in is learning how to fix a screw up in a professional way.
