Hi,
While adding a heater box cleanout I was able to add a cabin filter. I had to cut the filter some, but I figure it is much better than nothing, and I believe it will filter all of the air that comes out the vents. I did the heater box cleanout just like in this video I found:

First I drilled the hole in the box and found this:



There was so much leaves that the recirculation door wouldn't close, and leaves would come out of the recirculation inlet on the bottom right of the box whenever the door tried to move. I cleaned all of this out; it smells much better now when the heater is on and the airflow is significantly better as you would imagine.

Now I measured the heater core through the hole and found it needs a roughly 9 by 9.5 x 1 inch filter. Wix lets you look up filters by dimension on their website. The closest filter to this that was stocked locally and is flexible enough to fit through the hole is a wix 24815 or Napa FIL 4815. It does need some trimming which I will try to explain:



Please excuse mine for looking dirty and folded up, I had to take it in and out several times to trim and my silicone wasn't dry yet. With the filter in the truck, the arrow should point towards the heater core, and the felt ends running perpendicular to the pleats will be the top and bottom. Now there are two cuts to make. On the top right of the filter (top passenger side) you need to trim to clear a round protrusion in the box, if you look through the hole you will see it. The filter will be held tight to the core by this protrusion. Counting from the right, I trimmed from the 10th to the 20th pleat, approximately 3/4" down from the top of the filter, and halfway through the "depth" of the filter. Then, you may need to cut off the first two pleats on the right (I did both before taking the pictures which is why it looks like I cut from the 8th to the 18th pleats). Then you just need to install it:

I didn't install my plug when I took this picture but I assure you it will still fit through, you just might have to trim the left side some if your plug is way to the left, or at least tuck it behind the screws. Just take a light and make sure it sits flat on the right side and the top, and straighten it out with your hand if its not. Some of the smaller Nissans even make you squeeze the filter like this to install it. Here's the final product:

I installed the plug with 8-32 screws and nuts, some nylon washers, and some black silicone. The only concern I might have is the filter getting wet, but I am figuring that the evaporator drain must be on the left side of the heater core, otherwise the drain would surely have been plugged on mine with the amount of leaves in the box. Hope this helps someone, thanks.