This is an adapter cone set that I made to replace an inline duct fan (squirrel cage type) in a shower exhaust for one of our bathrooms.
The original was a wimpy and very noisy blade type fan that was part of a ceiling light fixture.
It had a motor using sleeve bearings and after a very few years the bearings made about as much noise as the fan blades.
Worse, they could not be effectively lubricated (believe me, I tried).
So I purchased and installed an inline "remote" squirrel cage blower some years ago.
However, even it was not all that quiet and only moved around 150 cfm of air, which was barely adequate.

Recently I was sorting through some things and came across an old NMB 5915PC-12T-B30 pancake blower I had forgotten I had.
It is, under ideal conditions, rated at around 240 cfm, a considerable improvement.
It is also MUCH quieter than the blower it eventually replaced.
You can still purchase fans like these on Amazon and other places for around $40USD.
There are 240 volt versions available as well.

Naturally, I looked around Thingiverse first for a 4" duct adapter that could be adapted to fit this fan (150mm) and found numerous things like this- https://www.thingiverse.com/thing:42490
This, and others like it, are exactly the right idea but it is for a 4" (100mm) fan and the largest of any other versions I found was only 125mm.
None of them seemed to be easily adaptable for this purpose.
So I designed the one in this posting to fit my 150mm fan.

I designed in a mounting loop which I used to suspend the assembly between the roof rafters in the attic with bungee cords.
That, in combination with some flexible ducting, helps to isolate any vibrations and much of the noise that the new fan generates.

Now it is far quieter and definitely moves more air (though it does sound a little like a jet engine revving up. :-)
It works very well..!

You can purchase commercial versions of this, but they tend to be rather expensive.
The less expensive versions ("duct helpers") will NOT do the job and typically use the very same cheap sleeve bearing blade fans as the one I originally replaced.
So they were not a viable option for me.

I hope this helps you.
Thanks for looking.