Have built a few of these, kit bashed a few more. As I recall, I glued a small piece of wood (or plastic) along the inside of the walls 1/2 inch (3/4 inch if I used corrugated cardboard for the roof) from the top. If the span was long (take a guess how long that is) I'd place a piece of wood or plastic across the span. After those pieces dry you'll cut the roof piece to shape, place it where it belongs and weather it. Or use paper towels or Kleenex tissue cut in strips to simulate tar paper. Add vents as desired. Have fun. John