Master Magnetics Flexible Magnetic Sheet - Non-Adhesive Magnet - Ideal for Car Signs - Strong DIY Magnets for Fridge, Dry Erase Board, Crafting - 24.5" L x 12" W - White Vinyl Front - 1-Pack

It’s a large sheet magnet from amazon. The kind where you can put it on your car to advertise. https://a.co/d/01jptacE Edit to add: if you decide to use the same one I recommend unrolling it and placing the whole thing on your fridge or some flat metal surface for a week or two, otherwise it won’t stay flat while you put on the stickers