So, assuming the photo gets rotated 90º to the left and the long side with the missing circles is the right side vertical, this is what I would do.
1. Make a regular sleeve of proportional width for the size of the quilt from the left point (green and orange circle) directly across to the right side. Leave both ends open as this will be the main hanging sleeve.
2. Add a second, bracing, sleeve along the short side of the triangle. Close the lower end of this one so the dowel doesn't slide out.
3. Then add a small ring to the back side of the top point.
Hang the quilt from the horizontal rod and use a small tack to hold the top ring in place. The bracing rod will keep the short edge straignt and prevent it from sagging over time, and everything below the hanging sleeve should fall straight without any help, again assuming that right edge is a vertical.