Our coke cooler inc is 35 high 21 wide and 19 deep
We have 30 inches of flexwatt on the back wall with the herpstat probe placed behind the flexwatt and a fan on the floor of the inc to push air around.
We only care about the temp inside the tub since the air inside the inc isn't always the same as it is inside the tub, at least in testing it they never were exactly the same.
We use two thermometers with probes, one probe inside a tub on the top shelf and one inside a tub on the bottom shelf and double check things with a temp gun.
There are several ways to place the probe in an incubator, this one has worked great for us.