Just installed one of EHS's airbox covers and I need to say it really helped to keep the dust out of the airbox. After a whole day in the dust, the inside of the box is real clean and the filter on the box should be good for another day or so. They did a nice job of making the covers seal well.
http://www.ehsracing.com/products/trx700xx_airbox_EHS.htm
http://www.ehsracing.com/products/trx700xx_airbox_EHS.htm