Please reply quick - something needs to be done tonight, maybe?

I had a ventilation area similar to yours on my old coop. I did wrap the coop with plastic and bungee cords, leaving a small portion at the very top of the ventilation uncovered during a very cold spell. I left their little door uncovered too, so they could either come down to their pen to eat/drink OR I could push a feeder/waterer into their coop through the door. I only had 4 chickens back then. It worked well enough. I just had to take unfrozen water to them 3 times a day.

 

Which direction do your winds normally come from relative to that photo? If winds are unlikely to blow through the triangles I wouldn't cover either one.

 

If push comes to shove and looks like the winds are blowing hard into the coop, tack some burlap if over the openings - it'll buffer any wind but still has some breathability. If you don't have anything like burlap then maybe some cardboard will do for one night, only in the direction the winds are hitting hardest... the idea isn't to prevent all air from coming through, but just to buffer it.

 

You could cover the side triangles with plastic, or cardboard. It sounds like your ventilation on the front and back would be enough for your 6 chickens. I don't think you have to worry about suffocating them, that would be very hard to do. But as you close up your ventilation, keep an eye on the humidity in your coop. If it should start getting wet in the coop, open up some ventilation as high on the triangles as you can for added cross breeze. You want to try to keep the humid air flowing above the chickens on their roost. Air flowing beneath the chickens is more like a draft and that would cause them to get cold.

Your chickens look like big girls, and I would expect them to do well overnight in any case. My nights are down to 2F and my girls are doing fine. Keeping the coop dry is the most important goal. Good luck.

 

If you are in the Northern Hemisphere, your prevailing winds will come from the west.
They do move around. I had a triangular opening on the south of an A Frame grow out coop for young meat birds. An extreme storm came in from the south overnight. It blew the roof doors open. Blew one chick into the next county but the other chicks who were met with cold rain all night, were fine in the morning.

I don't think you need to do anything. It may be a bit small for 6 chickens but I think the ventilation is good.

 

Excellent suggestion.

Burlap or something breathable is much better than plastic.

As long as their heads are below the vents, they are probably fine left open.