I have not used the Spellbinders ones BUT....I have gotten the rolled magnetic sheets from a craft store. Since they are rolled, they didn't lay flat once I cut them. I cut them into the sizes I need and put them on my fridge for a day or two and they lay flat afterwards!
I don't buy the rolled craft ones anymore for this reason alone. I stick with the vent covers found at Home Depot or Lowes. They are cheaper and then are flat!!!!