Are larger registers not an option?

If the registers mount flush to the ceiling, then the duct may just be the space between joists. If you can see the wood joists, then you can affix a pair of wire hangers to a couple nails.

Fashion wire hangers with a hook in one end using a coat hanger or any other stiff wire. Paint the ends of the hooks that stick out through the screw holes. If you put a little thought into the engineering, you should get a very strong grip that still makes it easy to remove the registers in the future.

Also, you can buy vent opening inserts for exactly your situation.

I don't think the next size up will fit into the duct. And I can't see any wood joists in the opening - it's a metal shaft.

Then you need the vent opening inserts.

... although I have been known to simply stick them in there with builders' adhesive. (Short story: I was the finishing contractor, and the developer was corrupt.)

You can try the tape idea, but in my experience, tape-based solutions were only good for short term.

The key question is do you need an airtight seal?

It's just a register, not some sort of filter, so I don't see why I would need it to be airtight.

Okay, so I need to look into getting these inserts. Thanks!

Take the hole measurements, and you should do fine.

i wonder why they are called "registers"? do people sign them? no. do they measure something? no. they just blow hot air. why are they not then called "quonsars"?

You won't be satisfied until we use "quonsar" the way certain little blue people use the word "Smurf" will you?

-- quonsarette