I used weather stripping to seal my end table humidor's doors and drawer. Something thin and able to be compressed. Found mine at the Home Depot. Something like the pic below but I think mine allowed more compression.

FWIW, I (personally) wouldn't worry too much about 58%. In the winter my humidor keeps around 61-62%, and around 65% when it's warmer. But, to each their own as far as RH goes.