Using this: http://www.amazon.com/gp/product/B00114LPC6?psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s00
I'm not sure it's the thickness of the wood. I've held the thing over the fan (outside of the box) and it still doesn't really spin.
I was going with no washers between the fan and the top of the box, originally, but there was no space for the fan to actually spin