Look at exhaust fans. Mount it as high as you can on the wall or ceiling to pull the hot air out. You can find them solar powered and with a built in thermostat.
I would hold off on the next 40 bags. Refresh the litter with a few bags here and there. Is there such a thing as Christmas tree mulch giveaways? Get a dump truck load.
Once again, well done.
20's and 30's hardly qualify as cold for chickens. They might be a little slower to come out in the morning but once they get moving it will just be another day. 110* on the other hand can kill them. They will stop laying and they will suffer. Build for the summer. You can always add...
Go with 2 sides open and covered in hardware cloth to maximize air flow. I would spray the roof as that's where most of the heat will come in. Pop in a exhaust fan if it gets to be too much.