When you forget your extra needle and want a three needle bind off, you sometimes have to make do. A crochet hook (yes, I actually own a few) can easily be inserted through both the front and back stitch
and be used to pull a loop of the working yarn through both stitches
which is then pulled through the working loop. End result is that lovely, simple and not terribly bulky seam you see starting to form on the right.
Or, if you wanted to be fancy, there are obviously more ways to bind off. For example, you could pull a loop through both the joining loop and the working loop to create a new loop. This uses a bit more yarn.
Did I mention that I finished up the front and the back
with only five meters of sock yarn left and am now facing endless I-cord. It is going to have to be in the solid color supplemental yarn that I used in the back.