Passenger side cowl, need to block it off

Don't want the air flow? I run my vent boxes wide open pretty much from May until September around here.

Um, I'd just duct tape it if that's all you want. It's not like it's high pressure or anything, and it doesn't need to be water tight (the originals aren't). Cut a piece of cardboard to fit the opening and then liberally apply duct tape to keep it in place. Will be fine unless you start to see some significant moisture.