EVERY SINGLE TIME that the game is over and people are leaving the field, stand next to the exit and just say "Barrel covers, barrel covers, barrel covers..." until everyone has exited. Don't let anyone out without one.
If someone removes their mask on the field, game or no game, tell them to put it on. If they don't hear you, repeat it until they do. Get progressively louder each time you repeat it, the most calm order being a raised voice, and the worst being rehearsing your 9-1-1 call and wiping fingerprints...
In all seriousness, be nice, don't be afraid to yell at someone if they need it, make friendly conversation between games (not too much talking, just a little small-talk), make sure to carry a microfiber cloth with you to wipe masks between games (after each game say something like "Anyone need a mask wipe, come over here!" loudly), and have fun doing it!