Before starting the seating area, we pulled a heating duct through the front wall, to provide heat to the living area of the bus during long drives.
We wrapped the duct in insulation, then covered it with the original housing and hooked-up the original blower fan (after a tune-up). We decided to build this heater into the seating.
Next, we threw together a quick frame, securing our joists sideways and reinforcing the undersides with angle iron. We chose this simply because every inch counts and we had a bunch of left-over angle iron, so what the hell? We gained another 2 inches of storage space and ended up with stronger joists than we would have otherwise.
The frame was then covered with ply in which we cut an access hatch should we ever need to repair the fan.
We built the bench frame just under 16 inches high and 15 inches deep and set the seat back brackets at around 120 degrees from the plane of the seat.
After testing the seats out with a board, we decided to raise the brackets up and out to compensate for back height and lumbar. 120 degrees was still a good angle for reclining, it just needed to start out just above the lower back. It also was helpful not to have to open the window when you lean back.
The whole thing was covered in plywood and we were done for the time being. Our raised seating didn't end-up being that much work, it not only hid the heat vent but gave us a place to sit, eat, store things and stow a mattress underneath.