That's coming together really well. Can't wait to see the finished product.
Use sheet metal. I just spent $80 on 14' of 20 gauge , 19" high. Plus it will keep with the sides that are on there now. Awesome idea.
Never dealt with this company, but the teardrop crowd seems to like this type of hinge.
http://www.lilbear.teardrops.net/parts.html