Assuming that the quarter is not easily replaceable. Like you can unbolt it and bolt another on. If it were mine I would cut out the rust back to unrusty steel. Then hand form or buy a full new quarter depending on cost. Cut out the section I needed Then weld in the patch panel. Spray the back with weld through primer if you can’t get to the back to paint after. Then epoxy primer, then body work, then paint. or just punch out the rust hole and fill with plastic filler then paint. It really depends on how much time and effort you want to put in.