Straight edges. 90° corners (T-Square provided as proof). Tidy attachments.

This is probably the 3rd iteration of this attempt because I wasn’t satisified with variations of attaching the live knitting to the knitted piece.  The one I finally settled on is: sl last stitch as if to purl, pickup edge stitch from the back, purl 2 together. On the next row, slip the 1st stitch as if to knit.  It’s a variant of Vivian Hoxbro’s method.  I found slipping that last stitch as if to purl provides the edge I like the most.  Your preferences may differ.

Attempt 1 has been ripped. This is the one I will be going with. (Although, I do miss the movement in the diagonal lines…)