The grounding bussbars I've seen have typically been HUGE ... perhaps 1" wide and 3/16" thick, simple copper.
While you don't need this much copper for a ground path, you do need that much for mechanical strength, and for the screws to have a couple threads engage the metal.
Hint: When drilling copper, you want to use drill bits with 135 degree 'split point' tips. When tapping, taps with spiral flutes work better. It's worth a visit to a local machine-shop supply house.
