If you werent sitting behind the Reds dugout I would say make a poster that says "Milton Sucks", but I dont think the other Reds players would like that.

:
"The Cubs Fan Next To Me Smells"
"Bartman Rules!"
Or you could dress up in Cubs gear and have a sign that says: "It Sucks To Being A Cubs Fan" or "I Think I Just Pooped My Pants"