That got me thinking...in looking at your first picture, what if you just took a flat metal bar and drilled four holes to match the two holes you have the bolts in and the two holes that hold the filter (on the outside of the case)? That would significantly spread out the force across the case. I think for it to work, the length of the two bolts that fit into the fins would have to be sized just right where they could be tightened down onto the bar. If the holes are not in the same plane, you could shim the bar by gluing on washers, or maybe it wouldn't matter....
Another option is, of course, to take the cover off and spread the load with small spreader bars then put the cover on and buttoned up slightly before attempting to loosen the nut.