4 hours of solid driving didn't melt a Twix bar stored in my cargo area. Out back, that vent was putting out some serious heat, as designed.
Make sure your engine compartment cover is on right. If not, I imagine it could vent hot air (not exhaust) into your cargo area. You should see and feel no gaps.
That's serious insulation and venting; the cargo area cover's only a few inches from the manifold. I don't think it gets past 90 degrees on the top side. Any other car, the hood is too hot to touch.