Connecting air duct to air register

Good evening all -

Sorry for the rookie question, but I've come across an issue with my air duct to air register which I was hoping the pros on the site could please answer. More than likely a very simple answer.

I currently have this set up which is a 7+" OD (OD is just a hair over 7" - maybe 7.125/7.2) air duct which I'm trying to use a 7" to 6" reducer to accommodate a 6" air register:


This is what the "elbow" or whatever that is looks like:


For whatever reason, the ridges on the air duct (it's an old set up obviously) will not accommodate the 6" air register - even with the reducer. The 7" side of the reducer will not fit properly to the 7+" OD air duct. Seems like HVAC work is never plug n' play. The "elbow" appears to be attached to a long sheet metal pipe. Does anyone have any ideas - any help would be appreciated! Thanks in advance!

Things I've tried thus far:
1. 7" to 6" reducer
2. Sheet metal (rolled and crimped) with self tapping screws unfortunately with no luck either.

 

This is the original air duct (similar from another room)



Any ideas guys?