I have not seen any metal covers except on some very old trailers. They do harden and are easily damaged with age. The covers are not all that expensise, even from RV dealers.
Problem is if you make it too rigid, you will run the risk of damage to the RV chimney stack and roof rather than an inexpensive plastic cover.
Amateur Radio Operator (KE5DFR)|Full-Time! - 2012 6.7L Ford Crew Cab Dually -2013 HitchHiker Champagne 38RLRSB - Travel with one Standard Schnauzer and one small Timneh African Gray Parrot