Finished installing the crossover box and the floor vent… I am pleased with the results:
Wall cut out. Wall anchors installed: Note: The air comes out from the other room on the left and goes back in and down the wall on the right, thence to the vent/register just above the baseboard.
Crossover box complete with filter retention screen:
2" Filter element installed:
Crossover box installed: Note: I added a 1/2" wide x 3/8" thick (uncompressed) seal between the box and the wall. Stuck to wall. Left secondary PSA liner intact so I can remove and replace the box to change/clean the filter. You can just barely see the edge of the seal around the edges if you zoom in and look closely.
All that remains in order to complete the air supply (intake) is to cut-in the duplicate 6" x 14" register vent near the floor in the other room.