I agree with QuiltnNan.....but reverse direction at the end of each diagonal quilting so you don't risk pulling the quilt out of "square."
Straight stitch quilting - like you do with SID - doesn't *have* to be "hidden" in a ditch; if fact, I personally prefer never to SID quilt. If I'm going to do all that work I want the world to see it!!
AND it doesn't have to be a diagonal design, it could be vertical or horizontal lines, too.
It'll look great!
Jan in VA