I know this is a little premature for posting but I just have to share this find.
We had scrapped a machine at work that had these AL. covers on them.
I had to wait until they went to the scrap yard to get them and they doubled the price they paid for them but, with the cost of AL down they were forty cents a pound.
They are 25 x 55 x 2.5 inches.

I grabbed a can of paint out of the garage and painted one black to see how much of a difference it would make to the heating affect.
It's below freezing now and in about twenty minutes the black one was warm to the touch and the white one was still cold.

Should be easy to finish these off for air heaters. Glue some panel insulation on the back and sides, add some glazing and mount.
Bonz