How do I make use of the extra space under the seat?
I've spent the last few weeks looking for a storage box of the right size that could be put under the seat between the battery and K&N's, to use the space made available from removing the airbox.
I finally found the right item at a K-Mart, after checking a few hardware stores (looking at mailboxes, plastic box planters, small parts bins, etc); it's an ordinary $2 file box for 4" x 6" index cards. The box is just the right size (well, it could be an inch deeper, but it works well enough). I simply removed the flip-top lid, drilled a hole in each top corner, and used some zip ties to suspend it from inside the frame of the bike, just below the bracket at the rear of the gas tank.
A 4" x 4"x 6" box isn't huge, but every bit helps. There's not a lot you can do with the space otherwise, since the drive chain and rear suspension are directly beneath that spot.