Not sure why they should be metal wrapped. Trivial to make from wood. Could attach separate pieces of sheet metal on each side or possibly paint with a metallic paint?

If they are decorative plywood for the sides, and a couple layers of thinner plywood for the bent portions.

You probbaly are going to need pressure treated plywood for decent life, and a very good paint job.

I would make them out of Azek, we do it all the time, & we ship anywhere.
http://thespeedofwood.com/Exterior.html

The columns aren't really arched. The brackets creating the look of an arch are known as "corbels." There are many on the market made of compostie materials like Azek, which, as calm1 says, is the way to go.