These HVAC Collars are much easier to install and seal than the commercial ones and cheaper too.  If I did it again, I would have simplified the design and square it off with 4 corners and 4 screws - but this worked just fine.

Included are collars with outside diameter exactly 4" and 6" .. which form a tight fit for flexible ducting.  I also include the Fusion 360 file .. but if you are competent with Fusion, these are trivial to make squared off from scratch.  These have a 4mm wall thickness, and I printed them with 3 wall loops with PETG.  They come out very strong.

In my case I'm only using plastic fittings for return air, so anything would work.  Not sure if PETG would cope with heated conditioned air, PLA certainly would not.