[Volusianator]

Anything thicker than 1/4" is not going to give you any more buffer for RH unless you plan to let your humidification run dry for months. Most humidors use veneer, 1/4 lumber is going to be just fine. Just be sure that if you plan on making larger panels, do your glue ups with a bit thicker stock and then run the entire panel through a wide belt sander. Local cabinet shops are usually willing to help you with this if you don't have one of your own.