I dont understand why you want to lower the humidity level,no matter what cycle your in your humidity is fine,even in heavy bud durring the last few weeks of budding a humidity level of 60% is not dangerous, as long as there is fresh moving air reaching the plants.
Unless the air in the grow box is still i wouldnt sweat it,if the air is still put a small inline exhaust fan on the top side of the box,then cut a passive intake at the opposite bottom of the box,the air transfer will lower the humidity.