question on adding extra return line

About 2 years ago I finished out a walk-in attic space (~300 sp ft). I added two cold air supply runs, but after time has gone by I realize I should have added a small return in the room. So, I've added the return vent in the room and I've run the 10" flex back to the main return trunk. When the house was built 5 years ago the original installers used a 16" flex duct as the main return as you can see in one picture at the yellow arrow the main return connects to the floor of the attic which is the ceiling of the upstairs hallway. The run is rather short as it comes out the back of the handler and loops right over the unit to connect into the floor. And there are no hard lines used. The red arrows just show the direction the new 10" return will run toward the new room.
Questions....
1. what is the best way to connect the new 10" return into the original 16" return. I'm assuming I'll have to build a rigid 16" pipe with a 10" takeoff and insert it somewhere along the run.
2. Where is the best place to put the rigid "T"? If I insert it right where the yellow arrow points to will it be too close to the original return in the ceiling and starve it a bit?
There just isn't much run to work with. I've added a second pic to show where the return line is connected to the handler and how it immediately starts looping back over the system.
Just looking for input and ideas to make sure this gets handled correctly.
Thanks!

 

It's possible the 16" is undersized (Say "DUH" but there are contractors who size flex as if it were round metal), and the new 10" affords an excellent opportunity to fix that. Please give us the model numbers of the air handler and outdoor unit. It'd be good to see an image showing more of the 16" return connecting to the unit, too.

 

You'll want to improve the flex supports too: The return and one supply are severely restricted, easily fixed by providing 3" wide sheet metal "saddles". And we don't hang ducts from ducts. Furthermore, the requirement to support flex on 4 foot centers is now 11-1/2 years old, having been reduced from 5 feet in February 2009.

Two other questions: Where is air the filter and what size and type is it?

 

air is lazy it will take the path of least resistance.
i would add a plenum on the return side and add the 10" and 16" to it

 

Definitely. I thought it'd be good to address any filter sizing issues at the same time, thus the request for M/N's, etc.