[GoatLocker]

Leaks through the "seams" or joinery of the cabinet should be pretty minor unless the cabinet is nearly falling apart. As Footbag says, you have to accept that winter requires more maintenance. Assuming the door seals well, you probably are not going to improve much.

Is this a custom made humidor or converted cabinet? I've seen some cabinets that had a thin plywood back that is unfinished on both sides. This can cause significant loss of moisture. Also, is the back panel securely fastened? Are there any floating panels that could be a source of the leak?

You could add some active humidification for winter, as beads don't have much capacity.