This is my 3' x 6' brooder in the coop. I put chicks in here straight out of the incubator or post office even when it is below freezing. This set-up is for later in the spring when it doesn't get that cold. The plastic is more for breeze protection than keeping heat in. When it does get that cold the plastic goes all the way to the top.
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I use heat lamps. I consider it very important to toss that clamp that comes with it and use wire or chain to firmly hold it in place. Do not use string that can burn or plastic that can melt. The biggest danger from a heat lamp is if it falls. Use wire or chain in a way that it cannot fall. Some people use heating pads or heat plates for the heat source. Those can work too if they are set up properly.
You can see a chimney I built off to the left. One heat lamp fits in that so I can raise and lower it for different times of the year. That also assures ventilation when the plastic is all the way to the top but if I were to redo this I'd think hard about putting the ventilation on the other end and adjusting heat by using different wattage bulbs. That would make keeping that end warm easier. In winter I use 250 watt bulbs, in summer maybe 60 watt, in spring 125..
In winter I put a second heat lamp in, wired firmly in place. This is mostly for redundancy, in case one lamp goes out.
One of the biggest challenges to brooding outside is the temperature swings. I've had it go from freezing to over 70 in less than 36 hours. The goal is to keep one spot warm enough in the coldest temperatures and one spot cool enough in the warmest conditions. I find that as long as the chicks have a choice they can manage where they need to be pretty well, even straight out of the incubator. Some mornings I find ice in the colder part of the brooder, you can guess where the chicks are. But when it is pretty warm outside they can and do get away from the heat lamp. That's one reason my brooder is this big, so they can get away from the heat of they need to. If you use a heating pad or heat plate that's less important.
The floor of the brooder is 1/2" hardware cloth. That way the chick poop can fall straight through, making it really easy to keep the brooder dry and clean. But in cold weather I use a piece of plywood to make a solid floor on the lamp end to better hold in the heat. When it is time to clean it I just tilt it up and scrape. The poop still falls through the wire. The plastic around the bottom is to stop breezes from below. That plywood floor helps with that plus it acts somewhat as a thermal mass and holds heat. In cold weather you want a solid floor.
If you are brooding below freezing one challenge is to keep the water thawed. There are different ways to do that. I put my water in the warm end where the heat lamps help keep it thawed.
I understand about wanting to brood outside. I have the choice between brooding outside and staying married. It is an easy choice. An option may be to brood in your garage if it is attached instead of in the coop. The temperatures may not get that cold in there, especially if you don't park a car in there where you are opening the big door. You still have to deal with all the dust they make but that might be acceptable.
Good luck!