Made 2 firecrackers with Nutella and Wheat Thins. Wrapped them in foil. Then placed the foil packet on my dashboard, with my truck facing into the sun on a 90 degree day for about 8 hours.
Got totally wasted. I would think that a concrete, brick, or blacktop surface would work as well. Even better if you cover the foil with a glass bowl to hold in the heat.
Please post if you give it a try.