I found a high flow fan so i replaced a 40mm low from with this adapter plate. I used M3 12mm cap screws to mount the plate to the existing housing then m3 13 phillips heads to mount the fan to the adapter.
Print with very low infill and 2 perimeters if you need to keep the weight down.
I printed with 60% infill and 3 perimeters so the weight would help cantilever the duct and hold it up off the prints.
the adapter has nut traps so you have to drill out the thin divider on the 50mm holes. Put nuts in the trap before you mount the adapter to your duct. they will not fit once it is mounted.
mount adapter to duct then mount fan to adapter, enjoy high air flow...