For a one off tool to do a few holes, you can just get a scrap piece of 1/8 or thicker aluminum and drill the appropriate hole for your selected screw.
For a #6 screw, drill a #36 hole in the scrap, countersink it (a little oversize countersinking works best) and then tap the threads.
Drill the #28 hole in your firewall and then screw through that into the dimple die you just made. Screw it in tightly and it will dimple for your screw head.
Van's, or someone, used to sell a tool similar to this to dimple spars on the older kits for #8 screws.