Prototype 2: (and possibly the final design)
This idea came to me last night after thinking about vintage AKG studio model headbands - I know it's not the same, but they're one of the most complex headbands ever and simultaneously seem to present a set of solutions to problems they naturally create by their own flaws, which increases the number of parts (which means more things
can go wrong).
I concluded that my first design was connecting to the headband in the wrong place.
With this design, the adjustment is done by the Hifiman headband (the headband never changes size - just the position of the earcups on the head changes) which naturally serves a wider range of head sizes, increases design simplicity and therefore keeps costs down. (these high quality pop-fasteners aren't exactly cheap for what they are)
The only possible weak link is the effects of weight on leather over time, although only the small pop-fastener part would need to be replaced in the event of material failure. I think if I produce any more, it will include a layer of fabric underneath to increase long-term durability.
A natural bi-product of this design is that the suede-like underside of the leather grips quite well - this is good if you want it; I know some people have complained in the past about the headphones wobbling on the head - this headband helps reduce that effect considerably.