I agree that heat soak is mostly a city driving/pulling from stopped problem.
I see lot's of people using plexiglass on other cars. It's clear, insulative, tough, and fairly easy to work with.
What if you painted the underside of it with some kind of reflective paint? Might stop even more of the radiant heat.
Might also consider some sort of flexible edging on it, that might conform to some of the irregularities and cut down more of the flow from underneath.
Just tossing ideas out.
