I think most people with big cooking grates just buy expanded metal when it's time to replace them. For the area that needs covering, it's the cheapest way to go. You will find a few people that are willing to pay a premium for a grate. Also the shipping to get it to them would be cost prohibited.
If you do decide to do it, make sure you make yourself some very flexible jigs and fixtures so that your setup time for manufacture is as short as possible. Time is money and I would think that profit margins would have to be tight.
"Of all the imaginary friends I've had, I don't think there was one that I didn't end up having to kill."
in seach of Umami