What are the measurements that yer trying fer? Entire width of the tail gate? Width of the rear of the truck? How big will the wing be? Where will it attach to?
If you make yer pattern from cardboard, you can git a good idea on yer dimensions. Making it yerself is possible with a simple bolt together system. Have you looked at other wing designs yet? There are tons of designs out there that you could copy from. Post up some of those designs that might work fer you.
My current material of choice lately has been aluminum diamond plate, aluminum angle, and slotted angle.