When warming up food on a hot tray on shabbat, most poskim do not allow one to put the food directly on the hot tray, but rather insist on a plate or inverted pot or pan be placed on the hot tray and the food be placed on top of that. Unfortunately, even using a shallow baking sheet creates a 1 cm air gap between the food and the hot tray, and the food just doesn't get that hot. Any ideas on how to get food hot with this setup? Hotter warming tray? Different type of baking sheet? Putting the food in a different type on container?