I've been doing some more experiments the past couple of weeks, and found a couple of ways to reduce the temperature in the trunk of my Ford Escort.

(See my previous posts for previous results.)

With no protection, the trunk area gets 30-35°F hotter than the outside temperature when sitting in the sun all day.

With an old reflective sunshade laid on top of the rear deck, the trunk temperatures were only 15-20°F warmer than the outside temperature. But that occasionally produced bright glare on the inside of the rear hatch, interfering with my view when I change lanes. And that's unacceptable.

So I laid an old white towel over the sunshade. And the temps were still only about 20°F warmer.

Finally, out of curiosity, I tried it with just the towel. And it went up to 20-30°F warmer.

While it wasn't as good as just a sunshade, the sunshade with the towel over it was the best acceptable combo. I'm sure the air space produced by the sunshade (it doesn't lay flat in the area available) contributed to that.

So that's the final result. In my old, battle-scarred little Escort, the towel over the sunshade on the back deck doesn't look bad, and keeps the trunk a little cooler. It also gives me an easily accessible towel, and we all know that's important!

A sunshade and towel will also keep prying eyes off what you have in your trunk and may deter theft.

Like most hatchbacks, there's a formed plastic deck there, that lifts up with the hatch, so visibility of the trunk area has never been a problem.

It only takes 10-15° off the trunk temps, but every little bit helps.

If you go to Home Depot, they have sheet of styrofoam which can be cut to fit a specific surface. Replace the sunshade with a sheet of styrofoam and use the towel over that if there's a reflection. . . or, line the inside of the trunk with the styrofoam, make an enlarged cooler of the entire trunk area.

Line the whole trunk is an interesting idea. Impractical with my skill set, but interesting.

But a piece on the back deck is a VERY good idea. I'll be checking that out!

While you're at Home Depot, stop by teh hand-tools aisle and pick up a Husky/Superknife that uses very sharp but expendable blades. A thin but very sharp knife is the only tool you need -- that and an idea of what you want to achieve.

I didn't mention the head slap I gave myself when I read your idea. I've worked with sheet styrofoam before and should have thought of that myself. I have a couple of "normal" utility knives, and an aluminum straightedge.

They have the reflective windshield aluminum cellular material at Lowes and Home Depot, you can cut to your size and maybe that will help too. I think it's called Radiant Heat barrier and it's not too much $$