Sorry that I misunderstood what kind of carnage you were trying to correct.
For that issue, both of the previous suggestions would work fine.
On the nutserts, they'll have a thin collar left on the outside, so I'd look at using some thin closed-cell foam tape around the inside perimeter of the cover panel, to seal it better and eliminate rattles.
On going up a size on the original type screw fasteners, one tip is to take a blunt round punch and judiciously tap the stripped sheet metal threaded hole, and slightly countersink it. This will slightly reduce the hole diameter, and it will also give a better "push" against the screw threads, instead of the hole edge being bent in the "pull" direction. Another tip would be to look at SAE spec fasteners versus metric spec, for a diameter size which is just larger enough, instead of a much larger next metric size.