Use a blow-off tube and don't worry about it. A 5 gallon batch in a 5 gallon carboy will blow the top out. Go to your LHBS and ask for a 3-4' length of blow-off tube, sink it in the neck of your carboy, with the other end in a pail of water. Once your vigorous fermentation has subsided you can replace it with an air-lock.
Dave:-)
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