Backstory My house's A/C filter grate had the metal lever that kept it locked in place no longer would stay in the hole, so it needed a replacement that would work. I modeled a very similar mechanism with slight tolerance differences for the part for the hole so it would work. I have no idea the brand of the original grate.
I designed it in Alibre Design.
Print Recommendations I printed it in PETG, it needed to be able to flex, PLA kept breaking, so I would highly recommend against using that. Print it flat, shouldn't need any support. I did 100% infill since I needed it to be strong since it was holding metal in place.
Hardware/Assembly Information It should pop into the hole if your vent grate is similar to mine. It is pretty close to the original design, so should operate in a similar manner. You just push to engage and pull on the part to disengage.
Usage Recommendations I have had my part in use for over six months without having to replace it, there have been a couple of A/C filter replacements since then without any issues.