I hadnt given a lot of thought to heat on top or bottom. With still air it would make a big difference, but with a fan or forced air not near as much. So one would think. Here is a plan for a home made incubator that comes from this site. I saw it a while back and fell in love with it. Im currently looking for a used frame to build mine in. Hope this helps or at least gives some ideas.
https://www.backyardchickens.com/web/viewblog.php?id=2636
https://www.backyardchickens.com/web/viewblog.php?id=2636