Here are some pointers:

# The interior wood must be able to absorb humidity well, which should be ensured by using a Spanish cedar lining.

# The humidifier should provide sufficient humidity to maintain a humidity level of approximately 70%.

# The hinges should be durable and ensure a low-friction functioning for many years. In quality humidors only quadrant hinges should be used.

# The humidor case should be well sealed and close tightly (not necessarily hermetically) so that no humidity may escape from the humidor. Then the humidification system will not require constant refilling.

# The basic construction of the humidor should ensure that high humidity inside the humidor will not result in deformations of the humidor case or lid and that the humidors will still close neatly after years of usage.

Prices range from $50 - $1000, I'd go for something middle of the way.