Here is a really simple design for a can cover. It does not keep a gas tight seal on an open can, but keeps stuff from getting in if you are outside or just want to transport an open can in a car, bike ride, etc.
The origin of this idea was taking a can of soda outside to the patio and (not noticing) a wasp dive into the open can.
UPDATED 06/23/2010 - The cover could come into contact with the can's contents to some extent, so if there is any question about ABS and food safety - place a piece of plastic food wrap between the cover and the can top.
UPDATED 06/26/2010 - The 'cover275.stl' file added below fits onto a 2.6" DIA. (66mm DIA) can. Layer of food wrap suggested between cover and can top.