I have done it where I sewed the borders together but left the last few inches of each border open. Then I attached it to the top and attached each border in succession at the ends so That it looked just the same as though you had attached them one by one. I did not miter the corners. It took a bit of figuring on paper to figure out the length of each successive border but it worked just fine and it was sure a lot easier than attaching them one by one!