like said , you dont need a fan - that is too fast drying then. Slower is better here.
Just make shure that you have something colder outside the box , you can place a cold bottle otside the box as a dew point and then you can see how the moisture is going from the weed to the bottle. How often you release the "watered" air from your box is one key value for drying speed, the temp/VPD difference beetwen Zone 1 und outside Zone2 is one key value, too.