use materials that are already there besides nails, rope, or whatever you choose to hold it together, shacks built out of plywood usually look bad and collapse. choose a spot that is hidden from chair view and doesnt have much traffic. other than that, there isnt much designing involved. im guessing this is the first shack youve built so i suggest keeping it simple instead of trying to build some extravagant structure. if you are looking for a challenge, make it jibbable.
good luck