Is the clearance problem with the sides of the cover, or with the top?
Depending on the amount of interference, if it's just the top, you may be able to use 2 gaskets instead of one, or if you don't like that, try making your own from a sheet of Buna-N rubber that you can order from McMaster-Carr.
If it's the sides, since its a cast alumunum cover, maybe a little work with a die grinder would do it?
Otherwise it's different arms like Larry suggests.