The helix changes the output, similar to a dimmer. A fan needs constant full voltage, either on or off.
I use two Johnson t-stats in line. The first is set one degree above the second t-stat. The heat is plugged in to the second t-stat. The fans are plugged in to the first t-stat along with the second t-stat. The heat goes off and the fans run unless the temp keeps rising, then the fans shut off also. I had a fan go bad and stick that wasn't controlled by a t-stat, and that area of the incubator was getting a little too warm.
Rich at Reptile Basics is very good, I bought all of my t-stats and stuff from him. I recommend him all the time. I have seen lots of incubators with the heat on the side or back, I know it will work. I just prefer to have mine below a false insulated floor to keep the heat away from the boxes as best I can. There is a gap at one end and a fan at the other end blowing up.
I don't think temperature is as critical with ball python eggs, since they don't really thermo regulate like a Burmese python.
Mike