I lay a piece of "Bubble Foil" insulation the same size as the pad on top of it. The Bubble Foil cost about 25¢ per sq ft (by the roll), and weighs about .47 ounces per sq. ft. Call a local home insulation contractor, tell them what you want it for, and ask if they'll sell you a piece. If they have it, most will. Even if they double what it cost them, it will still only cost you about $6.00.
So, I have $12.00 bucks in my set-up. For both warmth and lightness I'd bet on it against any down UQ and any expensive pad any day.