Drywall? Nope that would get ruined with the quickness. It doesn't take to water well and I spill a lot.
I am adding one on top (2"x6"- Once the liner is in) in the same spot as the bottom one. This is to prevent bowing out as I know the long 7.5' wall would. I don't think it will bow in the other direction so I am not bracing it that way. I also have metal L braces going in all the corners. Sometimes I think this is under built and then I see others and it seems over built so we'll see how it holds. In looking at other designs it should hold fine though. There is nothing else going on the outside for now. I may add more later but it's not needed and I will want to insulate it before adding anything to help with heating it in winter so it's staying bare for now.
I got the liner in today and will see what I can get done tonight. I'll update with some pics later.