Such childish questions... As usual!
Who said that the gasket should be round in cross section?
As an example, I did this like multi-body part;
also you can save this body and do the assembly (if you worry about mates - thats not problem)
I am not talking about the o-ring not being round, but I want to insert the o-ring in the assembly. I want the o-ring to follow the extruded cut path around the cover which is not circular.