What's a good humidity meter to keep inside the coop?

They're notoriously inaccurate. IMO if you really feel you need one, you probably don't have enough ventilation. Certainly if you ever smell ammonia in your coop, you need more ventilation, as it actually reaches harmful levels before human noses can detect it, or so I have read.

It's just a matter of opening up something at a high level in your coop that won't result in a draft blowing on the roost. There doesn't even need to be an opening for intake at a lower level. The air, humidity and ammonia will exchange at the high vent. Usually (unless your coop is quite small) this is enough to prevent draft.