Found this design by BigFish87 and thought it'd be great for a server rack cooling vent.
All it needed, for me, was a longer 100mm part for the 100mm ducting. I dropped it into Fusion to do this and the ducting fits inside the tube.
I printed this upside down, 100mm part first. The wood screws are just temporary. I plan on laser cutting a paper / card gasket for a better seal.
Polymaker 1.75 PLA