I've never raised Cornish X so can't help you with a lot of that but I did live across the state line in Arkansas from you and raised chicks outdoors in a brooder in February. I also used heat lamps. To me your biggest issue will be the temperature swings. You'll see it go from below freezing into the 70's in 36 hours. Your brooder has to be able to handle that. What you want is a warm enough spot in the coldest temperatures and a cool enough spot in the warmest temperatures.
My brooder was 3' x 6' and built into the coop. I'll include a photo. When it was that cold the sides were totally wrapped in that clear plastic. This photo is from when it was warmer. I still had great ventilation up high even when it was wrapped. You can see that chimney where I hung the heat lamp on the left. That kept the heat from building up too much on the warmest days. The heat lamp kept that side toasty warm but on the cold days I saw ice on the far side.
If you make a divider make sure they can't get trapped behind it away from the heat. For 45 chicks I'd use two lamps, both on the same end. Part of that is to spread the warm spot out but keep the far end cool. Another part is redundancy, if one bulb blows they still have heat. Make sure you secure the heat lamp with wire or chain so it can't fall. Do not use that clamp, those are dangerous.
I typically have around 20 chicks in here, all dual purpose. I put them in straight from the incubator or post office, even if it is below freezing outside. As long as they have the choice I find mine are really good at finding and using a warm spot. I don't know what your 4' x 8' coop looks like. As long as it is pretty open I would not worry about them getting where they can't get to the heat, but could they get trapped in nests? You may want to block those off.
I leave my chicks in there in winter until they are 5 weeks old, maybe a few days longer if the weather is pretty rough when they hit 5 weeks. Mine has plenty of room. You'll have twice the chicks and more than twice the room but some of yours will be Cornish X. I'm not sure how that will work for you.
You will have nights below freezing so your water can freeze. I put my water in the heat lamp area to help it stay thawed. I use a black rubber bowl so if it does freeze I can just knock the ice out. The rubber bowl will not break. To keep them from getting soaking wet I fill it with rocks so they can drink between the rocks and yet walk on water. They will poop in it so it needs to be dumped daily and refilled. That can be a pain when the rocks are frozen in the ice so I have enough to use different rocks while some thaw out.
My dual purpose chicks can get through the electric netting until they are maybe 7 weeks old. I don't know what age your Cornish X will be too big to do that.
I think it is very important they have good wind protection down where they are but they also need decent ventilation, preferably up high.
That's all I can think of right now. Good luck!