I went ahead and made a cover using paper to make sure I liked the fit. The original design fit ok, but the angle was less than the angle of the seat belt brackets, which meant that the air vent would not be flush with the top of the cover. So, I decided to change the angle. Here's mine:

I think this fit is a lot nicer. However, unbeknownst to me yet, there is a problem.

Here it is after cutting it out. The strange corner in the opening is to provide a small lip for the air vent flange. This will help the "leather" cover stay out of the way of the flange.

Five minutes at a local sheet metal shop, and it's all bent into shape.

The hole was cut with a standard 2" hole saw in a drill press.

Now I'm starting to glue on the "leather" cover.

On to part 2!