I don't have a walking foot either, and I sew straight(ish) lines with my regular foot. I just draw them on with a ruler. I have to admit, they don't look as neat as Karen's, tho.. Wow!
I love your square, and when I look at it, it already looks like it's "puffy" so I would stitch in the ditch in all the seams and then stitch an additional triangle in the white triangles, but going in a different direction. Like if the base of the fabric triangle is on the bottom, make the point of the quilted triangle face the bottom. Make sense? I hate trying to use words to describe something that takes 5 seconds to sketch out!!