Better to open the entire gable triangle, cover it with hardware cloth, then install either a roof extension or an awning.
Those louvered covers generally only provide square inches when ventilation needs to be measured in square feet.
Poor Dumpling has been having a hard molt.
Being a Brahma, she hasn't much tail to start with and she lost ALL her tail feathers at once. That little black fringe is the new tail coming in. :D
Venting needs to be up at the top both to remove ammonia and moisture-laden warm air (even in winter), and to keep the drafts well above the birds heads when they're roosting. :)
Here's an excellent article on chicken keeping in cold-weather areas...
Welcome to BYC.
It's great that you're giving your birds so much room! Overcrowding is the #1 mistake people new to chickens make.
I think you'll want to move that roost out a foot from the wall because they won't have room to assume their over-flowing and drooping nighttime position:
See...