FYI, I had a hell of a time with the rubber ones and with the cork ones, they kept squeezing out on me and leaking.
The perfect solution came from frpp. They sell metal valve cover gaskets, that have a little bit of rubber on the ends. They in fact come on all motorsport packages, however, they sell them seperately. They are pretty much re useable indefinately, and are fool proof against leaking, and don't cost too much. that's the only way for me.