Wish I could take credit for the idea, but I stumbled upon it on another forum a while back. What follows is my work though. My cabin air filter was old and had a bee or two stuck in it. I could order one easily enough, but where's the fun in that?
Thankfully the wonderful engineers at Toyota decided to give this thing an actual removable frame instead of just some lame channel to shove the filter into.
I use the 3M pleated filters in my house and try to keep them on hand. One was sacrificed to the cause. I laid the old filter atop this thing, marked two cut lines and used the cardboard border as my remaining two cut lines. Regular kitchen scissors got the job done, maybe make sure your wife is not around when you do this? Definitely make sure to be careful, the wires are sharp and poke
Once I had it cut out I just inserted it into the frame, and did some adjusting of the wires since they were bent while pushing the filter behind the frame's retention tabs. It's very snug, with no gaps, so it should perform just like the factory filter. Cutting carefully was the key, getting crazy with the scissors or trying to just eyeball it might not work well.
The last time I bought a factory cabin air filter it was around $12. I paid $14 for the 20x25 filter, and I should get at least four filters out of it, so there's some savings to be had with this DIY approach. There's no carbon on this, so I don't think it will eliminate any foul odors, but it is a good filter so it should remove allergens and dust at the very least.