This is a beautiful quilt! As others have said, it's fine to add borders just on the sides. How much do you want to add to each side? If it's a small amount (up to twice the width of the borders around the diamonds), then I would go with a dark border that reads as solid. A dark turquoise or purple would be nice. If you want more width, maybe something like this?
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Since the diamonds are based on 60 degree triangles, I would stick with that angle in the border. To me, flying geese (90 degree triangles) would look out of place. Whatever you choose, I think it will be great!