A blow off tube. Try to match some tubing, you'll want 2'-3' or so, to the hole the airlock would normally go in and shove one end of the tube in there, just enough to make a strong seal. It doesn't need to penetrate the head space.
Put the other end of the tube in to a half full container star-san solution, water, etc...
Make sure the container is well below the top of the fermenter, like on the floor near the fermenter, and the end of tube is resting on the bottom of the container.
Follow good sanitary procedures.