I was going to propose something similar. My thought was to take a piece of plastic or packing tape, and tape it to the lid where you are doing your buildup. Silicone around your rim, set the lid on, and let the silicone cure. Once cured, the silicone will peel off the plastic/tape and you'll have a perfect fit.
"This hobby is about 10% what you know, and 90% experimentation." - Lon Heim (DartMan)