Depending how vigorous your fermentation is, you might just be better off removing the cap, and just jamming the tube straight into the neck of the carboy. Without having your carboy infront of me, I can't say what size you need. But I believe they're relatively universalyl sized. My guess is around 1" ID or 1 1/8" OD. It's cheap at Home Depot. Run the end of the tube submerged in a bucket of water (same concept of an airlock).