I finally finished it!
It was a really quick and simple pattern by Catherine McMurray on page 38 of Handmade Magazine Vol 27 No 9. I omitted the top handle and pockets to make it simpler and faster. They simply weren't needed anyway. I also made the sling handle a LOT shorter to make it easier for Max to carry.
The only thing I'm not totally happy with is that the black thread shows up too much on the lining fabric. :-(
The bag looks a little big on him but the Suzuki books are quite large as is his foot chart which is currently still MIA...