When the new floor went in, they cut the hole for the vent to the exact same size as before. Which in retrospect explains the stainless steel plate. The hole is too big for the register.
For some reason, people like to stand on the register, which makes no sense to me at all. People kept standing on the register and it started to bend and crack the Marmoleum beneath it.
Fun fact! Registers come in very standard sizes. They are either 2 inches wide or four inches wide. They are never 3.4 inches wide, which is what I needed. I hunted and hunted and finally found a register at Reggio Registers with a two inch bottom and a 3.5 inch top. The only problem is that it slid around, sometimes showing the hole beneath. I thought if I could bulk up the base then the register wouldn't slide around and I wouldn't have a stroke every time someone decided to stand on it (but seriously, WHY).
One cheap paint stir stick from the Home Depot on each side, plus some 1/2" square dowels, stacked and nailed into place.
I'm sure this is how a real carpenter would do it. What else are those stir sticks for?
But it worked! Son of a bitch doesn't move!