I’m thinking about the initial plans for installing an ERV.  Possibly an UltimateAir 200DX.  My house has no duct work and running new duct will be a bit of a challenge.  My plan is to run as much rigid duct as possible with flex duct for the 5-10 feet before each diffuser to control sound.  UltimateAir recommends the Seiho 4″ adjustable round diffusers.  I just have a couple questions about installation.  The diffusers don’t have any type of installation collar for installation in drywall.  I’ve seen a couple elbows or straight 4″ connectors for wall or ceiling mount…I believe they are both Fantech.  Seems they are really designed around stud mounting.  I’d like to avoid removing lots of drywall, and am thinking that these diffusers really wouldn’t need the support of a stud for secure installation.  Just wondering if anybody has a recommendation for any mounting collars for the Seiho diffusers.  If there are recommendations for other good quality diffuser options, that would also be appreciated.

Also, for running 4″ duct in the wall and up from the basement through the wall bottom plate, what is the best practice for getting the duct up through the stud bay?  I think it’s mostly steel studs.  I’m thinking I could just run the flex duct right up the wall connected to a transition duct at the bottom plate.  This would probably result in a slight compression in a 2×4 stud bay I would expect.  
Thanks