Every panel is unique; made my grand daughter a Harry Potter quilt from a panel. I tried all the tricks known to me to get that panel straight and nothing worked. The solution was to cut a width of fabric strip of one of the border fabrics and then ever so carefully cut it length wise on the diagonal, creating two wedges. After sewing the wedges on the panel, it was as squared as it was going to get. Then I sewed the border strips to the 'squared' panel. The quilt was still a bit wonky, but, hey, it's Harry Potter and his world is wonky. Grand daughter was 20 when she got it and loves it.
As the saying goes, sometimes I amaze myself...