Oh yeah, something you can't see: I used heavy plastic tape and taped up the sides including the round duct holes. Two layers, one on the inside, one from the outside. I then cut out the holes again, leaving a large margin of now very stiff plastic tape all around - a good 10mm flexible margin of tape, essentially. I did this so that this would form a flexible plastic seal, smaller than the wire frame of the duct, that I could basically screw the duct into! So it's not just held in by friction, but also by actual material. Does that make sense?