I love what you did with your triangles.....it looks like it was planned that way from the beginning. As for another border, it depends on how big you want it to be. It looks good the way it is, has a finished look with the wider border. But if you need bigger, add another....but it will need to be at least as wide as the outer border there now. But let me tell you again, what you have done is great, so creative!