I usually lay a mercury thermometer across the top of my eggs. I got one for a few $ at the feed store and it’s on a plastic “card” so it stays in place even while the eggs are turning.
Well
I actually got several because I have a bator + a lockdown bator.
I grabbed some cheapie hygrometers from...
I have not tried to build my own ‘bator, but the ones that I do have (have purchased) are forced air.
They have a small fan in a cage that allows some space between the top of the bator and the downward facing fan.
One small hole above the fan
(1/4- 3/8”?) for air draw, and several much smaller...