Personally I'd go with a regular one piece gasket and probably put a very light coat of RTV on both sides of it.
If you cannot find a gasket, then you could use RTV, - but you after applying, make sure you only very lightly tighten the cover for the first, say, couple hours.
That way, you don't squeeze out all the RTV before it has a chance to "set up". Then tighten normally.
Either way would probably work. Hope this helps.