You could go to Napa and buy a rebuild kit. They’ll have them in there plus any other gaskets or parts you may be missing. If that Venturi seal is missing it’s peobably worth it to go through the whole carb and make sure everything is in place. Those little suckers are a real pain in the butt when they fall into the intake. I bet that’s what happened. In fact, look down there... you may find that it fell off when you removed the top of the carb.