You could drill a hole in the center of the lid for the rubber stopper and then use the airlock. You can usually purchase tubing that will fit the opening of a carboy.
Mine is 1.25 in. and it will seal the opening well creating a blow off tube. Put the end of the tube in a small container or Star San.
I actually wouldn't ferment 5 gallons in a 5 gallon fermentor as you will likely not have enough head room and possibly lose some beer to the resulting blow off.