Do it yourself

I had a dining room wall with a cold air return in it bumped out 2 feet. The return ran between floor joists to the outside wall and up into a wall register. The room is now cantilevered out into the atmosphere. They sistered two 2x8 joists to the outside of the existing joists and ran them out the 2 feet. I pulled up the floor board and knocked out the 2x8 they put in between the joists and opened up the return. The bottom of the new space is 5/8 plywood then outside air. Can I use a floor register for return air? I'm going to place a china cabinet over it that has ample clearance height. If I want to use a wall register, do I have to build a small header and take out the wall stud and sill that is in the way of the return? Can I use Styrofoam board to insulate the bottom of the extension? Does anyone have any other suggestions? Thank you,