Roper Whitney #5 hand punch, they are available new all over the net for around $80-90, mine I picked up used for about 20. Kinda hard to justify buying one just to fill a few holes, but you can also cut them out by hand, just is less precise and more tedious.
http://toolmonger.com/wp-content/uploads/2008/10/0927102-11.jpg If you have any sheetmetal shops nearby, or automotive body repair, they may have one where they could punch out the plugs for you. The factory sheet metal should be 19 ga, in Europe, 1mm is about the same equivalent thickness.
