I am in the process of trying to install a duct fan to force more airflow to a room furthest from the air handler. I have a 9" flexible duct. That is apparently an oddball size. No duct fans are sold for that, 8" and 10" fans are all I can find online.

It appears all my ductwork is cinched down on the collars and splitters with large zip ties. Can I get a 9" duct to cinch down on an 8" fan? I realize this is a bit ridiculous since an 8" duct fan will restrict the flow on a 9" duct, but I am hoping I can overcome any flow losses with the fan and actually increase CFM.

Full transparency - I actually already bought the fan as I was 99% certain the collar coming out of the plenum was 8", only to cut the outer layer of the duct and find the internal duct was 9" diameter (28" circumference).

Thanks in advance.

I don't know the answer to your question, but I'm curious about how well the solution works because I have a similar issue in my home. I need to force more air towards the master bedroom and decrease air to my office. Can the fan be set up to only run when the A/C turns on?

Yes, I wired mine to a 120VAC auxillary output terminal on my furnace PCB to a low amperage relay that controls the fan.

Golfer friend used this to wrap the fan to fit the duct
Amazon

That's exactly the type of solution I needed. That exactly provides the 1/2" buffer I need to get the 9" OD on the fan. Thanks and tell your buddy thanks!