I would take the work apart clear to the first border that buckles. If you add on to it without fixing the problem, it will just be getting worse.
Measure the top (the piece where you need to add the border) across the center and cut your border (or adjust your pieced border) to THAT dimension. If your top is different from the pattern, then the border needs to be different too.
It's worth the effort and you'll be much happier in the end.