I made an angle finder with two rulers and a clamp and fastidiously checked the angle of the opening against the doors as I was sanding the stiffener into shape. After I glued and clamped the door, I checked the angle of the rear of the door and found it to be still sprung open a bit. That’s why you see the tape stretched over the bucket to the rear corners. Remarkably, the stiffener seems to be able to hold the shape of the front and rear edges.