sealing return passage for outside package heat pump

am looking at a recent install of a Trane package unit heat pump, return is built into a hallway with louvered grille and 1# filter media frame. The question concerns best practice of sealing the passage to the outside heat pump.
There is a 20ft length plywood framed box that runs the length of one bedroom wall, providing inside the room with a closet space. Inside this framed box, at the very end of it, is a flexconduit tying it all in to the outside concrete slab where the Trane system sits.
The flexconduit is very short in length (under 1.5 ft) but I am wondering if it would make sense to line it inside the 20ft run with some insulation material to reduce possible suction from the walls and worst case the attic.
Here's a rough diagram of the system
House hallway -> Return w/1# filter -> 20ft run built on one wall of one bedroom -> 1.5ft flex -> Heat pump outside house
The Trane runs great, keeps house dry and comfortable but would like to get opinions on using some type of easy to install materials to line the inside walls (all 4 sides) of the 20ft space after the 1# filter and before the 1.5ft flex conduit
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