That is just stunning. I have heard that her patterns are quite challenging to say the least. I think you are on the right track with the borders you mention. I would plan it so the last colored border hit the edge of the mattress. In the "More Black?" portion (the 'fall' that hangs off the edge of the mattress) I would think about scattering some small tumbling blocks to echo the ones in the quilt, especially if you have some left over fabric from the ones in the quilt. You could piece them into the border, or applique them, which would be easier.