I have the REAL gaskets on my rocker box covers and they do just fine. However, the reason I mentioned the gascolator situation is because I personally experienced the failure. I used the same material (cut it from silicone engine baffle material) to make gascolator gaskets. I flew the airplane for about 6 months with them, experiencing no problem.
Then one day I went into the hangar and smelled gasoline, and saw a blue stain about a foot wide beneath the airplane. I expected to find the gasolator quick-drain leaking.
(Keep in mind, that I am the guy who keeps telling everyone to shut off their fuel valves when the airplane is stored.) So to confirm my curtis-drain was leaking, I turned on the cockpit fuel valve and dumped about a gallon of fuel onto the hangar floor before I could stop it.
The silicone gaskets had warped/shrunk and would no longer seal.
I have used cork successfully, but I decided to use the genuine Cessna part from now on.
The genuine Cessna Gascolator Gasket PN is: 33-199-6 Gasket . Two are required. www.cessnaparts.com
has them priced at $10, but I have purchased them locally for $1.50.