Rain barrier on top of deck

I have a section of my deck where I don't want water to drip through the spacing. I can't put anything under my deck to catch the rain so I need to put a rain barrier on top of the deck boards. It is a small section, maybe 4x4, where there will be little if any foot traffic.

Anyone have an idea what I can use? I can't find 1/4 inch pressure treat plywood.