I wouldn't risk any ball python eggs by using an incubator that's iffy at best. Crested or leopard geckos, or corn snake eggs are one thing, being fairly tolerant of incubation problems, but If you are breeding bps, there's nothing more important than having a very dependable incubator. Whether you build one yourself or buy one, it should have a reliable digital thermostat and hold a consistent temperature (within ±0.5° max) no matter what temperature or how much variance the room it's in gets. Why spend years and a lot of money to get to the point where you have viable ball python eggs just to skimp on the incubator? You might save a couple of hundred $$$ up front, but you lose that savings as soon as you lose a single co-dom egg to a temp spike or other preventable problem. If you are just messing around and have a single clutch of normals or normals/pastels or something than sacrificing the acceptance of a little risk for saving money is fine, I guess, but other than that it's too important, imho.