Lowes & Home Depot both sell portable structures similar in size and scope to what you are talking about. They have examples of them built and sitting on their parking lots in my area (North AL). Visit a store with a camera and tape measure and you'll have plans in short order.
Be sure to take closeup pictures of all the nonsquare joints and note the cut angles, etc. Best of all, when you get stumped or you don't have the right picture detail, just stop back by and refresh your memory.
