Somewhere I read about a guy who put a plastic bag over the power head, put the cowling back on and used spray-in foam…
Sounds like a good recipe for a fire later on.
That was a good friend of mine, Art Seselberg, who did that. It was on an OMC 20 horse from late sixties I think. He told me it worked great, and no fires. Of course after the foam set up he had to carve some of it away so the cover fit and had some air flow around the powerhead.
Other question: OMC went from using basically open metal covers and inlet silencers, to sealed fiberglass hoods used as intake plenums. Are these "tuned," such that missing foam, or extra foam, effects the output?
Don’t use spray foam. It will clog up your jets continuously. Stick with a non friable material, such as the vinyl septum or closed cell rubber mentioned.
In 2013 I added 1/2" thick Soundcoat acoustic insulation to my 1990 Evinrude 15. The original insulation had crumbled, so there wasn’t much to make a pattern from other than the cowl itself. Four panels did the trick. The adhesive is so powerful that you only have one chance to position it. So far, it has worked well.