I am so sorry - I know how frustrating it is finishing a quilt only to find out that something is not quite right.
Not knowing how "cupped" you quilt is, my first step would be to try and block it with steam on a flat surface or a light spray of water.
So, for all my hanging pieces, I always add a pocket at the bottom of the quilt for a thin piece of wood. This is intended to keep the quilt flat and straight on the bottom as it hangs. If you were to do that to your quilt, you might be able to as juliasb suggested - the wood at the bottom would stabilize the entire bottom and if you were to clip some sort of weights to the pocket or the wood itself, it would distribute the weight to the shorter sections while not distorting the quilt edge. I might start with the quilt dry and apply just steam from a steam iron as it hung. Then if that did not work try a spray bottle with water and at last resort wet the entire quilt.
So - just as an afterthought and thinking out loud here- if it is not cupped badly, you might be able add the pocket as I suggested at the bottom and then add one or two near the center - slide in some thin slats of wood and let it hang. A 55 inche Quilt is a fair size quilt with some weight to it - with the pockets and wood slats it might hang just fine after awhile ....