I use a medium size, black roller suitcase (think air travel).
Ever the boy scout, I like to be prepared, and bring two of almost every piece of uniform equipment, plus toiletries, rule books, pump/needle and most anything else folks on this site have suggested may be of use as an oficial at a game.
Overkill? Possibly.
Have I used everything I carry? At one time or another.
Am I a little bit "teched in the head? Probably.