I've never used the ONA gel before so I don't know how well it works. And since my opinion is welcome, I would say to make yourself a carbon filter. Since you are running 4" ducting the supplies would be inexpensive, around 40 bucks. In YouTube lookup "diy carbon filter", there are many vids you can see to get an idea of what you can do that would work for you.

The small crockpot of Potpourri is a good idea. I'm guessing your doing a cab or closet grow indoors. Might want to look for indoor plants that you can place outside of your growspace to throw people off about the smell. At any nursery you can ask any employees for plants that give off a fragrance. Mint is my favorite. Smells almost like budding cannabis......