Heres an easy option!
I went to a local print shop and asked if they had a hot laminator and whats the largest they can do!
Then i had them get some card stock paper i believe its 12 x 20 and had them Laminate it. I just tape it to the car with blue painters tape and its on and off in two seconds!
I then use paint marker/dry erase - just the largest tip marker i could find.
I never have to change the numbers, cause i pick the same number every time.
oh, print shop charged me 15 bucks for 6 of them...two for me, two for my buddy and i got an extra two incase someone wanted to go autoX'n or Drifting with me!
-J