I'm no board expert and haven't used 28 gauge... but 7 years ago Jayboy Combs walked me through building my own big board. I used 24 gauge sheet metal (like RPD recommended to you) and it only took one motor to power the entire field. I still have the board, and it still runs good today. I don't know if 28 gauge will work better, but 24 gauge definitely works.

Good luck with making your board!

- Don