Ah yes, I missed that. The next time just cut your wheels in half so they are in good proportion!
Regarding the fan - no, mini fan is too strong and will inhibit growth. You just want the air to be in constant motion - not to be a storm in a box. I have an awesome solution: Aquarium air pump! the pipes are on the bottom of the box so they also push all this collected moisture back into the air. Works very well. Cheap and reliable too. Also in pet shops, where you get these pumps, you can get all these accessories to split the hose and to throttle how much air flow you want on it. It's fantastic. (just make sure to get a strong enough pump if you want to do multiple hoses to multiple boxes)
Here's a photo