Where, exactly, is the location of the leak? Are you certain it's at a screw hole?
Gordo's correct re resurfacing the cover. A warped inner cover or a crack at that screw hole can also create the problem. Multiple gaskets will not avail you not; a gasket is only intended to make up for manufacturing tolerance spread.
The root of the problem is right there in front of you. Look closely and you'll find it.