Thanks! Yeah, I'm using the internal fan right now. It seems to do the job, but I haven't tested it with full boxes. I set it up over night set at 89.0 with the fan going and this morning my low-high said 88.7-89.2, but I saw that those fluctuations were happening at the beginning when it was getting up to temperature. Once it was going for a couple of hours it seemed to stay right at 88.9-89.1. I have some extra 12v fans wired up in case, but I should be ok.
You can use any heating element you want as long as it can get hot enough without burning anything. Cable would probably be best so the heat is spread out around the eggs, and not just coming up from the bottom of the unit.