If you have access to metal working tools like hydraulic bender or even a manual metal worker, not that hard to make your own. Have made tunnel extensions for years out of aluminium from old road signs. There are different thicknesses of aluminium, the best ones I have found are the bigger signs, thicker aluminium. Our city/county used to give them away because they had so many used ones. If you bend them with the paint so it doesn't show the other side is usually the same color as your tunnel if unpainted. If you have a painted tunnel just bend it with the paint on the outside, sand it for your black paint to adhere and you can't tell it's not factory.
