I have been using these now for several years since I discovered them at Walmart of all places in the stationary section, and they have made the old #64's obsolete for me.
They are marketed as book markers, in a relaxed state seven inchs long and they are approximately 1/8th inch square. Have used them on everything from the typical 40 sized trainer to my forty year old Super Buckineer that has a twenty inch wing cord. The smallest is in fact my Apprentice which I sometimes use for training. On that one the seven inch band works perfectly with just a single double back.
John