Bill, i'm sure this one is going to be another "Home Run". Since no zippers, can velcro be used? It would seal the entire attachment area so that there are no drafts - something that buttons and snaps wouldn't do in a strong wind.
If velcro can be used, you'd need to use the "loop" portion on the main part of the garment so that it doesn't snag any clothing you're wearing, etc. I'm sure you'll come up with a better idea.
Can't wait to see the finished product.