How to patch a hole

[Brake Pad]

Your best fix is to cut a piece of steel bigger than your hole. Place it on the inside and weld in place. I have a 110V Lincoln mig welder that can run as a mig (with shield gas) or with flux wire, and recently did this exact patch with 0.030 flux core wire. Then fill with bondo, sand to preference and prime/paint as you like.

 

If you need an easier (aka no welding) patch method, then make your steel piece with several small holes drilled in it. You can add a few small holes to the roof too if you like. Then affix the patch in place, from the inside, with JB Weld. Now just spread your bondo across the entire area and allow the filler to ooze thru the holes. It may take several applications, usually slower & thinner applications will work best. Just be sure when sanding that you don't get all the way down to the steel. Leave a layer of filler to hold the patch in place. There is even some fiberglass mesh you can apply to the inside to give the filler something to form onto on the back side. Bondo is surprisingly strong filler, and will do fine, so long as its not a highly stressed area on the body. It's not fancy, and may crack in a couple years, but you were asking for alternatives...

 

Yep, what he said !!!!