"The guy at Hennessy suggested I get one of those things you stick on the windshield of a truck to keep the sun from heating the inside in the summer. Think that would work well?"

That is what I use to lower the temp I can stand IN ADDITION TO my underquilt. It's lighter than my CC pad, & has easily gotten me to 10 degrees below my "normal" 30 degrees with my underquilt. Probably in reality, it makes the 30 degrees more tolerable, for 30 degrees is "what I can stand" not what I'm comfortable with.

For me, the (reflective) truck pad is instead of added insulation to the underquilt is because I don't have anymore room in the pack, & the truck pad fits nicely Outside AND dosn't care if it gets wet. I think it's about 1/8" thick & reflective on both sides. I wouldn't sleep on it on the ground, but it adds a tiny bit of padding & works fine in a hammock.