I have the same set-up you do, and I used a piece of rubber trailer step pad material, cut it to fit the INSIDE of the airbox overlapping the opening by about an inch on the top and bottom and about a half-inch on the sides. This stuff has adhesive backing, but I used a little trim adhesive to insure that it sticks. Wipe the inside of the box with some acetone to de-grease. Works for me...Just ordered my EHS mudder cover and was wondering what the best way to block the stock opening was.
2009 Brute Force 750i