The only solution I have ever seen was slitting thin metal sheet and stamping the louvers out, just like the real thing. If you were the risk taking type, maybe vac form each louver, trim, and fit to a rectangle opening in the hood. You'd have to carefully align each one, and fair them into the hood. I don't see how the slits could be added afterwards. Man, you have some crazy-hard work ahead of you! Best of luck, and please post photos, as I'm sure whatever solution you work out will be invaluable to others.