What's the best way to add more ventilation that's easy to close up for winter?
Open the gable triangles and extend the roof to keep the rain out.
You don't close them -- roof-level ventilation is needed both winter and summer to allow ammonia and moisture to escape.
Here's one way to extend the roof: https://www.backyardchickens.com/ar...e-overhangs-eaves-of-a-shed-coop-house.76599/