consider breaking down some cigar boxes and using them to line your treasure chest (I did this last year on one of my traveldors (18 count).. worked like a champ.. I just found some boxes that had roughly the same dimensions.. and then cut them down to fit.. it took 4 different boxes for me to get panels to fit my top, bottom, and 4 sides.. but I got it to work..
for a seal.. a simple strip of 1/8" weather stripping all the way around either the top or the bottom.. and youll be sealed up as well as most humidors.. especially if the top has any weight to it.. if you want to get really fancy you can use a router attachment on a dremmel tool to make a 1/8" wide x 1/16" deep channel along the surface to put the weather strip into.. but I dont think thats totally necessary (although it would look nicer and likely seal up slightly better..)...