There are many questions that would determine the thickness of the wood for a humi. Most cabinetry uses 1/2" to 5/8" plywood. 3/4" would be way overboard.

What style/size box do you want to make?
What are you making it out of?
Are you going to laminate it?

Personally, for a large box style humidor, 3/8" to 1/2" plywood, laminated with something nice looking, sealed with a few coats of polyurethane, and lined with 1/4" Spanish cedar would work great.

If you need any help or have questions feel free to PM me.

spiffy