Geoff,

Back in 2000, I replaced my vinyl backed headliner with a product
called Heat Shield, It was an insulation liner with a vinyl finish
material placed over it. It looked like bubble wrap and it did a
good job of keeping our cabin cool with an additional reflective
shield. I'm not sure they are still in business, but you can
goggle for them. I dealt with people from a boat show that
represented them, Hotwire Enterprises in Madiera Beach Florida.
Their phone was 727-217-9809. The material was held to the hull
with strong double sided adhesive foam strips. Over time, some of
them have loosened, so adding supportive molding would be a good
idea.

Sherwin