Call around to sheetmetal shops and see if they have a Rotex punch. If you find one ask them to punch out some slugs to the size you need. If you have a step drill you can bore out the holes to a larger, more common size.
Yes, there are other punches that can punch out slugs but a Rotex is simple and fast with no setup required. Should be able to get all you need for a pittance. You'll just have to hammer them flat, the will come out bent. The punch is used to get a clean hole, not produce a clean slug.
I just don't like the idea of you scabbing a lot of square pieces of metal to the back of your firewall when you can make a nice, clean job out of it.
