Where should I install duct take-offs on a return plenum box?

I have a 5-ton system. My goal is to add an 8" return duct to a 15×9 guest room that has an 8" supply but no return air. When I went up to the attic to review my options, I saw that the installers had attached two 16" ducts directly to the air blower (Trane P0V0C000M50SCA).

So I bought a 19.75" x 20.25" x 30" plenum box with a liner to install horizontally on the right side of that air blower to extend it, then reroute those two 16" and add an 8" take-off for the duct I'll be pulling to the guest room ~35ft away from the system.

Where on the plenum box is the best place to install them? Is it OK for the two 16" take-offs to be placed and aligned directly across from each other on opposite sides and the 8" on the top or is there a rule of thumb to follow when installing (eg. staggering them)?