Track numbers on car for DE event

This is what the Chicago PCA recommends:

"Your local instant sign shop can make magnetic, vinyl adhesive and static cling numbers. Static cling numbers can blow off at high speed. Magnetic numbers should be taped to prevent movement.

For track events the number must be at least 6" tall on the sides and 8" tall on the hood, contrasting with the body color. Autocross only requires the number to be displayed on each side. The three-letter region abbreviation is not to be displayed."