Must just be the old 6 qt then that are flat, everything else Ive always used rubbermaid but they are getting damned hard to find in my area now for some reason. I typically use a single length of heat cord underneath and the shelf is routed just deep enough to allow it to be flush with the surface of the shelf so do you think there will still be ample heat exchange? Ive mainly used cord due to the expense and its worked fine with them flat bottoms, trying to avoid the cost of tape if I can continue using cord and it be sufficient lol.