The Cessna Mod Kit to replace the "thermos bottle cap" is SK337-40C. It can be ordered from any Cessna dealer. Last time I bought a kit it was more than $500. I recommend it because it adds a plate at the tank filler opening that has a raised lip to eliminate water getting into the tank, has a smaller opening to preclude getting a jet fuel nozzle in it and adds vented caps. I have a few extra parts for sale but not a complete kit.
Don Nieser
Commodore Aerospace Corp
6221 Commodore Lane
Oklahoma City OK 73162
405-722-4079 phone and FAX
405-503-4686 cell
nieser.02.337parts@juno.com