I second the tweaking thing - even with a bought incubator you have to get it warmed up and check temps, and adjust.
The best bator is one that works
The key components are a fairly well insulated box of some kind, a thermostat and a heat source (usually a light bulb in a homemade bator). A PC fan makes it a LOT more stable.
Most people include a grid, and wire covers to protect the chicks from the fan and light/heat source once they hatch.... and a glass insert for a lid.
My bators are both made out of 34 Qt rubbermaid coolers. Temps took several days of watching to tweak the thermostat.
Humidity is controlled by the use of a water tray in the bottom, or I can flood the entire bottom if needed, and holes around the top I can cover with duct tape or open as needed.
Ventiliation is important too, and so I have several small 1/4" holes drilled around the bottom of the bator, below the wire grid, for ventilation.
Good luck!