Nope, I was right the first time! Geez, I really suck at comparing odd shapes. I double checked the image, then traced the duct onto graph paper and manually counted the squares. They both matched my original calculation. I think part of the problem is that the kidney fitting has a female end which is larger than the minimum duct size, so it looks bigger. To get around this I cut the graph paper shape out and placed it in the duct to verify it was the right size (tracing it was very tricky).
OK, now I just need to fix that hookup problem. I added a mounting bracket and a valve, but I'm a little worried the valve will leak too much since it's hard to keep the thin sheet metal flat enough. I can always replace the valve disc with something thicker.
I figure it'll be easier to see it without the duct and such, so I snapped this installed pic. I kept the leftmost tab, but bending it back would require smaller fingers than I have, so the right one is now held on by that bracket. I just insert the left tab behind the shield then angle it into position and insert the bolt. It's pretty solid, surprisingly. Must be because I made it as tall as I could so it's snug under that black bracket.
Phew! Fun, but enough for one day.
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