Sweet Warbonnet, I am doing the same thing! I am making the end triangles a full 5 feet wide at the bottom, is that what you did? Also, I am going to sew a zipper in the other side (the side not sewn to the tarp), any ideas why not to do it like that? I am also planning to use a fabric 'door' like schrochem was talking about, I think I like the open top better for ventilation now that I see yours tho.
BTW I am modifying my GG tarp for this project.