Going to be hard to avoid unless you can figure a clamp system that will hold it.
As for the rust you can try this. No welding, cut or grind out the rust. Treat the area with some type of rust inhibitor. Make a metal patch that goes behind the hole. You need a automotive body adhesive. Either rivet (flush rivets work best) or use screws after the adhesive is dry grind of the heads flush fill with body filler, prep and paint.
Check this out for more detail and a flush fit panel method. Both of these work for rotten rocker panels too.
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