I would use stainless. Look in the yellow pages for metal fabrication and you'll find someone that will give you stainless for a lot less than you think. It will take a few calls before you find the right shop but when you do it will be very simple. Give them exact measurements and you won't have to do anything but stick it to the wall.

I purchased a 1/8th inch thick sheet of aluminum, custom cut, for my backsplash and we used a few flat plated bolts to attach it to the wall. It's heavy enough to stay perfectly straight and looks great ,and cost $200, compared to the $1600 the stainless steel would have cost. The thickness is the key. My only concern is I'm still looking for something to clean it with that will still leave my planet intact.