Trularin is dead on, cork is a much better material to use with sheet steel covers.
Like everyone else has said, make sure everything is flat/true and lightly torque your cork gaskets. When the
oil hits 'em they'll swell slightly and seal off any small gaps.
At least until your sheet metal covers start distorting again...
Cork really works best (with any cover) but most people lean on the wrench to hard. Nice, spongy gaskets are what you need here.