You may be able to make a border - pieced or plain - and use your zipper foot to attach it to each side. A better idea would be to put the border on before sticking the 'diamonds' onto the canvas. I started one of these in April, had to put it aside, and now it sits, waiting. Also I find it's easy to knock the diamonds off as they don't stick all that well which is why I suggest using the zipper foot.
But they are beautiful.