After a few little tweaks, I knit the customer's yarn (lovely soft Malabrigo!) into this:
The cables on the back turn into the shoulder straps, and I just love how it looks:
As I worked on these, I wrote the pattern, and after some help from my test knitters, it's ready for purchase at Ravelry!
I will have the Seller's Version (an inexpensive unlimited lifetime license) available at Their Own Knitted World when we stock again.