In my case, used an HDPE panel to fill in where the window used to be. This setup is for my laser cutter so uses a 4" hose but could easily have been any size round hole or moderately complex shape. I rough-cut the hole, then used a pattern bit in a router to trim it to circular. Naturally since I have a wood shop downstairs, track clamps were the obvious choice to hold it securely to the window frame. The black outline behind the window frame is weather stripping. The fitting is standard dryer disconnect but a blast gate would work here as well....how would I fill in the rest of the space around the blower?
(Yes, I do know the hole is positioned too high and the corner robs the exhaust of capacity. I have since built a riser so the laser cutter, external blower, and vent are all now at the same height and that bend is gone.)