I have made a few pyramids/Tipi's recently, and I can't seem to get the door zipper to come out very well. I have basically used a flat felled seam going from the ground to the apex. Then I try to add a zipper to one of the seams that stops a distance from the apex.. The problem is where the zipper starts. I can't come up with a good way to transition from the flat felled seam to the zipper since the seam is "interlocking", how do you essential unlock it without making a mess of the seam? Should I be using a different seam?