Boveda calibration bags. I don't know how you would seal the sensor inside but I have one very good calibrateable hygrometer. I calibrate that with the boveda kit and then set that by the sensors in the tent.
The inkbird and AC units should have a +/- adjustment. Cal those to the hygrometer and you will be as close as possible to accurate.
The boveda calibration bags are on Amazon or direct from their website. It is just a strong plastic bag with a humidity pack. You could easily make one if you have humidity packs.
I do find that you need to let the hygrometer sit in the calibration bag with the pack for at least 24hrs to be absolutely certain.