One thing you can do is to buy a roll of the rubber like stuff they use on roofs. It a peel and stick membrain that you use under shingles. It's sometimes called an ice& water dam.

You peel and stick it to the sheet metal. It sort of like the factory sound deading sheets but you can buy it at any home improvement store.

As for using expanding foam. As long as the foam won't get wet you are good to go. It it can get wet it will hold water and will rust the metal.

Ask me how I know? I used some on the hood of my van, inside the ribs. I now have to replace the hood. It was easier to buy a used one then fix the rust on the original.