I've got a vent cover that fell off at my ex-wife's business. It looks like they just screwed it into the ceiling tile, which obviously wouldn't hold. I can back it with some wood or something but I'm guessing there may be some sort of clip or something for this. I'd guess that if the duct was cut correctly the vent cover would screw into the duct. But the hole is bigger than the vent cover.

Is the grill round or square/rectangle?

They have 2x2 grills for drop ceilings.

It's a square. I guess it's likely 2X2. I'll look for that.

I was referring to the shape of the supply register.
For the round diffusers, you can get a ring to go on the back side to secure the grill. However, the ceiling tile will bow over time due to the weight of the fixture.

Using the 2x2 grills are the way to go.
https://www.google.com/shoppin...YKHUP2AHMQ8wII6gIwBA