Round AC/Heater vent replacements?

[denemante]

I have a 3300. According to Mermaid (the AC/heater maker) - vents are 6 inches - and a few others are smaller. I bought a generic 6 inch round replacement (which I planned to paint) - but it didn't fit. Seemed real close. One of these (5-6 inch?) vent covers is highly visible in my boat, and the fins were 70% broken and the whole thing had turned yellow. But yay, in my forward bow/bunk to the side (not easily seen), there was another of the exact size - still yellowish, but no broken fins - I swapped them for aesthetics - then when trying to adjust, broke two fins. Completely brittle. Anyone replaced these things with a drop-in replacement?

BTW - Mermaid says a "6 inch" supply must be within a few feet of the return - mine is under my dinette blowing towards stern. Strangely, many pics of 3300 show no such supply duct vent here...

For those who come ahead, mine were taped in with actual shiny reflective silver "duct" tape - I happened to have some. I had to cut/peel it off to remove, then used the tape to seal the seams back up. Super sticky stuff. Unfortunately for me - this vent cover swap didn't work out as noted. It's a simple job - if I could just find something that fits! Yeah, it would be "easy" to hit Home Depot and just find something close, grab a sawsall, adjust hole size, etc. It would be WAY easier if I could just find a drop in though...