Nissan won't show it on their parts diagram, however, you will see the bolt and spring nut on some aftermarket cabin filter instructions. I thing the original intention was to have the bolt and spring nut, as the case and door seem to be made for it, but they later decided to save a nickel and just rely on the plastic clip on the case. Not a problem; you can just go to your local hardware or auto parts store and pick up what you need for about a buck or two. I got mine at Lowe's in their general hardware section. I used a Hillman brand "M6-1.00x25 Body Bolt," which looks something like this:
http://thumbs2.ebaystatic.com/m/m2sY6Nd ... 1Q/140.jpg
And a 6Mx1.0 spring nut, or "U-nut," which looks like:
http://images.oreillyauto.com/parts/img ... 11-007.jpg