You did not say - is the border too long for the side - or is the side too long for the border?
Either way, if one eases in an inch over a long span, it would be fairly inconspicuous. Then you could attempt to shrink the longer span to be "smooth" with the shorter span.
I would fold both components into eighths, then match those points, and then sew the pieces back together, easing in the fullness. Most machines feed the bottom layer a bit "faster" than the top layer, so the "longer" piece should be on the bottom.