I usually grab for same shaped or close, from any Dodge Metal in the Junk Yard. But I'm sure there is probibly a correct answer on the right way but the same shape or close Dodge metal in the junk yard is all I know.
Good Luck!
they have it a lowes. about 20x30" for less than 10 bucks. i think its more like 20 ga. maybe thats because mine is all corroded away. they have multiple thickness there (lowes). Im doing that right now also