I'd just buy a rubber one at the hardware store if the trip to the LHBS is out of the way. It won't hurt for a few days so you can tune in the kegerator and try it out. They may even have a silicone or neoprene washer there. I never looked.
If your takin' your girlfriend out tonight
You better park the car well out of sight
'Cos if they catch you in the back seat trying to pick her locks
They're gonna send you back to mother in a cardboard box !