I built something similar but only 10' high. Built platform attached elevators then flipped it upside down. Attached legs, crossed braced all 4 sides and then flipped it upright using 4 wheeler. I made 4 corner post using 2- 2x4's shaped like an L and screwed them to each corner. Attached 2x4 horizontal members for siding to attach and used mtl roofing.................pretty light and simple.