No hygrometer necessary if you use a Boveda pack. The plastic can't be seasoned as it wont absorb any RH, nor will it lose it if it seals properly. As mentioned, crack the lid off for a few seconds once a week or so to get some fresh air in there and you're ready to go. The Boveda pack is somewhat self regulating so a hygro isn't necessary, beads work well too.
However if you were just to use an old cigar box as your initial post stated, you'd most likely end up with dried out cigars as the wood from the box would want to get moisture from somewhere. Now it'll get it from the Boveda too, but also from your sticks. Not to mention, cigar boxes aren't made to have that great of a seal, but like mentioned above, an inexpensive Tupperware container will do the trick very well.