Get friendly with sign companies, mis-prints happen all the time.....check around election time.....don't take signs that is a no-no....but the signs with errors can not be used and the shops will usually part with them cheap or even free....they are also all weather.
Good idea. You just gave me another idea, if I can find it buried in my shed with all the other junk.
I've had a roll of aluminum valley flashing in my shed for years. That would make a good waterproof target as well. I could cut sheets out and scribe one inch squares on it?