I can hardly believe seeing your new thread. Just yesterday I finished the exact same mystery quilt with my guild in Calgary! Mine is finished a year late, but that's beside the point
I too was stuck on the borders wanting to use up what I had already used in the quilt. Here are 2 photos of what I ended up doing. I hope it gives you an idea. I love the colours you used. Aren't mystery quilts just amazing when seen side by side?
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Also, I flipped the border corner blocks because I thought it would be very difficult to make the points (the 3 little white triangles) nice when attaching a binding over them.