I'm not sure how big it is, but I've used RTV sealant to plug a bunch of holes that I have to run wires and it works perfectly fine, if the area is small. There's also any number of different types of apoxy, JB Weld, etc that work wonders at any auto parts or hardware stores.

If you had to cover a large area, I'd probably cut out a piece of metal, apoxy or JB Weld it on there, then RTV seal around it.

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