The registration makes a great suggestion to put the car number "meatball" decals on magnetic sign material.
I started looking for that, was curious how to make sure I get the right stuff. Certainly sign material blowing off at speed would not be good.
Looks like lots of choices here: https://www.magnetsource.com/collections/flexible-l1?page=1&filter_index=
"Commemorative Event "Meatballs", pair
• 12.5" x 24" door decals • Specially formulated removable adhesive to not damage paint • Individually numbered to specifically identify your car on track and while on the road tour events • Pro Tip: Bring two sheets of precut flexible magnetic material with you. Apply your decals to the magnets, then use your meatballs indefinitely in the future. Highly recommended for all participants in open track as numbers are required."
I ended up buying this, I hope 24 x 12 is close enough.
https://a.co/d/01zyijrh