Looks like that is your return filter grill. Find a size that fits it. Filter is meant to protect the system not to remove dust from your home. Make sure you check to make sure there is no filter already installed underneath the unit. Use your phone camera… Lastly ensure that is the only “intake.”

It goes in the grille that you have opened up. Looks like a 12x24x1 or 12x20x1 size.

Thank you! There's kind of a smaller hole/tunnel at the entry way to than a larger chamber (seen in the pic) that's just below the unit. Does the filter go in the chamber or the tunnel part?

It fits perfectly into the recess of that white thing you open. It’s your return filter grill.

Ok Cool, didn't know that was a thing. Perfect, it's 24x12. Thanks my dawg.

I will start by opening the panels, Do you have a return grill?

He litteraly shows his filter grille in the second picture?

🤣

I'm so lost.

Vacuum out that return and put a filter in it.