I would use 1/2" closed cell foam for the whole thing, and grind it down in the back. Don't put any sew foam over it, just smooth it out to your satisfaction and then glue down the vinyl over the top of the closed cell foam. Closed cell foam is very firm, and it will be easier to keep the vinyl smooth than with softer foam. You need to scuff the surface of the closed cell foam on both sides anyway, or it won't accept the high temp top and trim contact adhesive you should use to hold everything down.
No one lives forever, the trick is creating something that will.