No question the better the seal on the case, the more efficient your in-case humidifier will be. Michigan winters make it imperative that the case has a reasonable seal on it.
I'm sure my instruments would be sunken and cracked if not for being in quality cases with case covers over them. I also use a dual humifier system (both home-made) That uses quality sponges and regular (about once a week) re-hydrating.
Just about this time of year (until about mid October) I get to take a break from it all
