Thanks for sharing the design, I am putting the finishing touches on one in my back yard. I had one lesson learned to add and a couple of mods that went well. For the lesson learned, be mindful of your carriage bolt placement. A few steps after making the frame you will need to install the floor. Four of my bolts partially interfered with the floor boards, I ended up notching the floor boards. Other than that, you can add an additional 2X4 to the floor, it helps keep rain out of the bottom. At the kids request, I added a floor to the bottom and enclosed the rear of the structure with a wall. Finally, I opted for a corrugated metal roof.