Installing MaxxAir vent cover - question

I had MaxxAir covers installed by the dealer, but I ran into a different problem on my way to Alaska one summer. It never gets totally dark, and there's a vent directly over my rear bed. I stopped in Dawson Creek, BC at Canadian Tire (like Sears). They had stacks of black plastic "cardboard". A knife and some double sided Velcro kept the "Midnight Sun" out when sleeping, but still removable during the day. I still have them, in case I find a CG that leaves lights on at night.