It's not the box that keeps the humidity, it's the humidification devices that are placed inside the humidors.
If you wish to use a humidor for storage, I suggest you "season" it or else it will dry your buds in no time. It takes about two weeks to properly season a humidor. I keep my cigars at 72% RH, which is probably too high for ganja. Fortunately, many places sell humidor beads and they come in a variety of humidity levels. Also, the more air that is in the humidor, the more your buds will be prone to drying out.
A very nice alternative to a humidor is a cigar jar. These are made from acrylic, have humidification devices within, and have an excellent seal.