Since my jar already had openings on top it made this project very simple. And instead of essential oils I used editable oils for candy making and they worked just the same.
Fill the jar about 1/4 of the way full with baking soda.
Add 5-10 drops to the baking soda (I chose peppermint) depending on how strong of a sent you want.
Just give it a shake side to side whenever the room needs freshening up.
Update: I came across an image of a parmesan lid on a ball jar. It does indeed fit (on a regular jar not the wide mouth ones) and makes the cutest most simple air freshener. No need to make holes just replace the lid and add 1/4 baking soda and essential oils. Could also be a cute way to freshen the fridge instead of leaving the baking soda in the box.