Personally i don't think you can get any better than a Genuine Ford rocker gasket. Most of the time they leak because 3rd party gaskets have been used, and when tightened it bends the rocker cover.....

There is a bit of a nack to doing them but i have fitted a cheapy £2 rocker gasket before now and had the cover off a few times with no leaks. Put your rocker cover on a flat surface and u will more than likely see its no longer "true"