Metal around wall register boot for trim
I don't think they're made anymore, but I heard them called Register Mounts, Extensions &amp; Collars. I looked for replacements a few times &amp; ended up with nothing to end up wire brushing them back to bare &amp; spray painting them, good as new.

If no-one else here answers or has an avenue to get any. You can make your own quite easily &amp; of much better quality...in the size that you need, which might be the real problem. I'd suggest Aluminum Angle for it's workability &amp; corrosion resistance, but you can go with steel or C-channel or J-channel.

You'd probably need about a 4' length for each one &amp; can either attach the last corner with a tab on the inside (not on the outside like the picture) or use a corner brace with countersunk (if desired or needed) screw, bolt or pop-rivet face holes.