I had a scoop plug I made for a snorkel scoop I had on a Challenger that I towed on an open trailer. It may not have been the prettiest thing and absorbed some water that got through, but it worked.
Cut some foam out a little bit bigger than the scoop opening so it provides a snug fit and reaches inside a few inches. Then cut a "plate" out of some type of thin, somewhat flexible plastic that will sit on the hood, but be a little bigger than the scoop opening. Glue the two while pressing the inserted foam on the inside and plastic plate on the outside together while the hood is open. Clear silicone may work. Just don't get any on the hood/scoop.
I don't know what exact materials would be best, but that's my fabrication suggestion.
I also knew a racer who had a plexiglass insert that blocked the opening. But it was held in with little brackets & screws.
