Without any sort of border, you are going to cut off the tips of the boats. Personally, I would continue the sashing with the cornerstones and then do a simple border. If you stay with only an edge sashing, cut it a bit wider so that after it is bound, it will be the same width as the inside sashings. If you add an additional border, it does not need to be very wide and could be any of the fabrics used in the top, depending on what you have left and what the top 'tells' you it needs.