I have the original type of rocker cover which i had spent a lot of time stripping and repainting it a while ago and decided it was now a good time to fit.
When I fitted it I noticed that although the two nuts at each end were getting tight and clamping the gasket at the ends the central section of the gasket was obviously quite loose.
I guess after years of use and over tightening the cover has distorted taking on a “banana” shape!
So I found a flat surface which happened to be my garage floor and with a hammer and a piece of wood hammered the flange back to shape. To check it I placed the cover on the bench over a torch to see how much light I could see under the central flange area. The gap soon reduced from nearly 2mm to almost nothing so refitted the cover …….. and so far so good.