Could buy some cheap ones like this to compare
I make these filament storage canisters to store my filament. I find it better to make them lid down. I then put these humidity meters in them and some desiccant.
Buy reputable fillament to start. It should come shipped vaccum packed with desiccant. The filament should be spooled cleanly and have a rated dimensional accuracy of no more then ±0.05mm. Snaps can happen if the extruder gets binded by the fillament being too large, or if it is tangled.
If your filament has already absorbed a lot of moisture you will have to dry it.
They look like this, brand likely doesn't matter (they're all super cheap), I found a 12-pack.
But then, you wouldn't be able to properly measure the humidity in each container.
Amazon has these hygrometers in a 4-pack for $15, and they appear to be panel mount:
Veanic 4-Pack Mini Digital Electronic Temperature Humidity Meters Gauge Indoor Thermometer Hygrometer LCD Display Fahrenheit (℉) for Humidors, Greenhouse, Garden, Cellar, Fridge, Closet https://www.amazon.com/dp/B07GNMKYCZ/ref=cm_sw_r_cp_api_i_9MYJFb64AHEJM?_encoding=UTF8&psc=1
Might be I could mount one in each container with adjustable venting like you say. I think that could work.