Jokes

Here is the situation I am dealing with: https://imgur.com/a/mw5BEZqOur furnace has always had a grill on the side of it to heat our basement (12”x6”). We recently began finishing our basement and put up a wall to separate mechanicals and living space. I (admittedly) didn’t plan much ahead of time and just left an 8”x14” opening in the wall. I realize I may have to resize this but if there is a way to keep the opening that would be even better.My current thought process for making the connection would be to use a flat piece of sheet metal to close off the existing rectangular opening and then using 6” collar to 6” pipe to some sort of connection to the wall. I found a 6” round to 4”x10” rectangle at the local big box. Is that what I want? And then resize my opening to better fit that size register? Or is there a better option?