I'd rig up a blow-off tube ASAP!
The easy way is to use some 1" tubing, sanitize it, and jam it into the carboy opening. You may have to go to Home Depot or the hardware store to get the right size that would fit. Then, stick the other end of the tubing in a bucket of water with some sanitizer in it, so that the end is covered. That way, the co2 can get out but fruit flies and other stuff can't get in.
The other thing I'd do is keep an eye on the temperature of the beer. An active fermetation can get really hot, really fast! If the beer temperature is too warm (above 70 or 72), stick the carboy in a cooler of cool water and add frozen water bottles as needed to keep it down.