My brooder is cheap, easy to assemble, and can be replaced for every new brood
get a clothes dryer box from the appliance store. Tape the ends securely and tip onto one side (this box is 29"x43").
Cut a hole in the top, about 4"inside the edge
Line the bottom of the box with a thick (contractor grade) plastic bag or heavy plastic sheet. Tape ask around the edges.
For young chicks, I use puppy pads on the bottom instead of shavings, just laying clean ones on top of soiled ones as needed.
When the chicks are older I switch to wood shavings, scooping them out as needed with a dustpan. I often put chicks in an outdoor pen during the day, and move them back into the box at night if it is still cold out. You can make a roost with a board across the box, and for older chicks I cover the top with an old baby gate to prevent escapes.
These boxes have a square opening in the end, I just cover this inside and out with packing tape to make a window.
The top can be covered with wood scraps or cardboard to help hold in heat, this one is in my (cold) garage so sits on a piece of foam insulation.