Hi Got2, the impression I got is that you're trying to keep your grow area as basic possible, There area a couple of DIY ways if you don't want to use a dehumidifier or put it in an air conditioned area.
1) feed cool air into your box, cooler air can't carry as much water vapour as warm air. As the cool air warms up it'll decrease the humidity because it's distributing the water vapour into a larger volume.
2) Take some good sized drywall scraps and remove one side of the paper surface, Put the drywall into an oven at 300 degrees F for about an hour. Put one scrap into your grow area and the other(s) into a sealable bag, then remove and replace every 48 - 72 hours. Gypsum (drywall) is a natural desiccant and literally grabs water vapour out of the air, it'll absorb almost 40% of it's weight in water vapour until it reaches equalization with its environment. The reason to bake it, is to boil off the water vapour.
Neither of these methods is as effective as a dehumidifier or air conditioning but it'll help somewhat.