Ok on norms Plans

http://www.gbfans.com/image.php?url=/images/equipment/plans/norm_1.jpg

item C on the material list ( Styrene plastic 18“ x 24” sign) since they dont make signs like that anymore and i dont really want to pay to much to order it online, I believe its used to make the spacer.

would the aluminum signs work just as well?

Or do you guys have a simple solution that would give me the same result just with a different material (one I would like to easily pick up a hardware store.

Any Help would be greatly appreciated.

what I am doing is using 1/8 plywood and cutting the same shape twice and then useing wood blocks to space the two apart then the hollow part i will cover with the sign plastic