I sew my pieced borders with a straight seam
and place that seam where there is already a visible seam within a block against the border. The eye already sees that seam; the border's seam just continues that visual....IF anyone really notices it all all.

It's enough to sew the binding on the diagonal as far as I'm concerned! ;)

Jan in VA

Tartan has a good idea. If you need to lengthen it, put a decorative block or two on each side or whatever. That sounds like fun. Any way you do it, I'll bet it'll be pretty.