Okay Andrew . . . we are going to take up a collection to get you some nicer numbers and class designation. Or send you to art school so you can use Painter's Tape so it's more pleasing to the eye. Just kidding but it really looks professional to have good magnetics on the sides of the car. In a pinch, get the magnetic material directly from your local sign maker and cut out numbers yourself. Spray paint them using the flexible interior paint and you're good to go.
www.solotime.com has a vast selection of numbers and kits. They're not that expensive and you can choose colors, type fonts, and sizes. Minimum size for autocrossing should be 10" but I recommend 12" for best visibility.
Congrats on your autocrossing and keep 'em coming. There's also a monthly magazine, North American Pylon, that has most of the nation's events in it. Great reference material as it also has the upcoming events. I think a yearly subscription is $24.
Cheers,
Mary Pozzi