If you don't have enough head space for the krausen, just use a blow-off instead of an air lock. Lots of folks use 5 gal. carboys as primaries and use a blow-off and it seems to work pretty well for them.
I usually have just over 1 gallon of head room left in my bucket and have had no problems so far, but then, I haven't done any truely big beers yet.
And now we go AG!
On Tap: Nadda
Planning: Extra Special Bitter