It depends on you access, doesn't it? If there are lots of bricks (very nice to build with and look at) or cinder blocks (easy to build; needs help visually) left over from the building project, you could use them. Ditto lumber and corrugated metal sheeting. Cinder block is durable and low-maintenance, but bulky. Metal sheet on a frame of steel pipes or wooden 2x4's gives the most interior space per square footage. If properly rust-proofed, it lasts longer than wood, but is less attractive. OTH, it can be painted a cheerful colour, which might change every three years. Cedar is inexpensive, attractive and quite durable. Personally, I'm partial to cedar slab, which is readily available and very cheap where I live (waste product of a Mennonite sawmill). A little harder to work with, as the boards are irregular in width and thickness, but the very randomness of it has a certain charm that factory-precise boards lack.
