I used 22 gauge galvanized steel roofing panels. Completely flat, numerous colours to choose from, multiple lengths to buy from and best part is them being steel magnets will stick to them! I paid $40/sheet, but in Canada, so the US should be near half that price I'm sure.
Panels overlap and use 3M double sided panel adhesive for joining, and then used Pin Drive Anchors (I have concrete walls) to affix the panels along the seams. it's pretty sweet looking as it's all white, then you have the 'riveted' look from the pin anchors. Completely wash down proof due to being roofing panels!
I would attach some pics, but currently in the middle of Africa so internet is horrible.
Pin anchors were from McMaster Carr..
http://www.mcmaster.com/#concrete-anchors/=11847gf