The light trap in a film holder is usually a kind of springy brass comb covered with black felt. I'm not sure where you can get this brass comb material, but I'm fairly sure it's out there, perhaps for some other application. Maybe Sandy can offer some suggestions.

You could also cannibalize an old 8x10" wooden holder for the light trap. I think Graflex holders are usually made with screws, so they're not too hard to take apart. You could reuse the comb, cut it down to size, and replace the felt.