Another option would be to add other win conditions, like the game automatically ends after an hour. That would add some strategy too- if your team is up and the clock is starting to run down, you could devote a couple players to blocking the other teams seeker. But you would have to be careful because then the other team would have an advantage scoring goals, and could sneak back into the game that way. With a few minutes left, unless the score was very close, the game would turn into a mad scramble for the snitch since that would be the only way to make up the score difference, with maybe just a couple people playing defense to keep the other team from getting free goals.