One thing you can do is remove the quilt when you are halfway done and re-mount the quilt from the other side. In that case, I would start from the middle (where you left off) and work to the bottom.
If you want to try the above method, you might want to spray baste the sandwich together (ideally before starting on the first half, but you could also just spray baste the second half before mounting). This could make it easier to keep all the layers lined up.