Passenger side cowl, need to block it off

I had to remove my heater box due to a leaking heater core. I forgot the last time I had this box out it was 12 years ago roughly and I did some quick-fixes. I need to spend time doing fiberglass repairs, painting the flappers, and adding new seals. In the meantime I need to drive the car.

I spent the past 30 mins searching this forum and didn't find anything. Does anyone have any ideas how to temporarily block off the passenger side cowl? There's nothing to mount anything to it seems so I'm scratching my head on this one.

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