Sealing up valve covers

Good info on the curved cover rail, never thought of that. Cork gaskets always seem to work. Doubling the gaskets smart idea, too. I might use Gasgacinch to glue the gaskets together. Or Aviation Permatex, which works well for threads that weep, Aviation can be hard to remove so use judiciously and on threads that aren't ever removed. Both you can glue gaskets together on the bench on a flat spot & light even pressure w/ something flat on top. Cast covers good for that.
Use too much aviation & you'll have a messy edge to clean.