I'm going to pick up my seats at the end of the month, and I had a question to all those who have build risers for the back row of seats: how did you build it? I'm guessing I'll use 2x12 for the sides and front, and then for the top deck some 1/2" plywood. But what I can't figure out is where to screw the actual seats into (they came from a theater, so they have 2 screw holes on each side; the distance (width) between the sides of the chair is 20". Should I just do another set of the 2x12s as joists under each chair screw point, or do I need to use bolts instead of screws. I'm also thinking of carpeting the riser; any thoughts?