The first 3 I made were your standard cooler size... y'know, the 48 qt size that you lug to a tailgate party.
I put the light in the bottom, and the screen over it, just like the directions. But the last one I made out of a very large fish-shipping styrofoam box. It's 33Lx14Dx11H" in size... very long. So I put the fan in the righthand corner about 1" from the top, where it would have space to pull air. Then the light bulb to the left of the fan, so the fan would blow on the lightbulb. Then the thermostat to the left of the light a little ways. I have the thermometer on the floor where the eggs are. I used hardware cloth to section off that end of the box so the chicks couldn't get in there, and put a large peanutbutter jar in the corner to hold water. I put down rubber shelf liner on the floor of the cooler box, and put the egg cartons right on top (I have a 1502 incubator, so my homemades are really only used as hatchers now). Holds about 60 eggs in cartons.
I swear, once I dialed it in, that thing held the temps like a champ, and I've had excellent hatches in them. This one does cycle on/off a little more frequently, but there is only about 0.5° swing instead of 4-5°F.
I've not had good luck with the lightbulbs on the bottom, because I sometimes miss when adding water, and have blown a few of the bulbs...
Not fun while you're in the middle of a hatch. You need to be able to get in, get out, and stop fussing about.