[brettweir]

I'm using one of those emergency blankets now and they're not great, but they do the trick. There's tiny holes in the blanket from the way it's folded, which doesn't reflect anything. Also, it's super thin and easy to tear. Those folds can also create hotspots, but if you tape it tight enough you'll be fine.

I'm pretty sure mylar has the best reflectivity though. Flat white is close behind.