I couldn't let another Christmas go by without having them done so I got my act together on Monday and whipped them up using my own tutorial :)
While I was making them though I realised my zipper insertion method was a bit of a PITA (I think I was feeling particularly anal about getting things perfect that day), so I tried pinning the zipper in with the zipper under the fabric and the fabric right side up. I just felt for the zipper with my fingers and pushed it around until it was lined up directly under the seam. And it worked just fine! No need for fiddly basting, hooray!
I also changed the method for doing the box corners. I cut off a 1x1 inch square (measured from the seam line, not from the edge of the fabric) then folded the fabric as before and still putting the pin in the seam to ensure that they'd line up and sewed 1cm from the seam line. This made it easier to sew the seam at a 90 degree angle to the other seams whereas before I had to be quite careful not to get it wonky.
So that's another job done. And today I want to sew up the Counterpane Clutch that I started about two years ago!