I searched hard and eventually found these G hooks which are exactly the same as the ones Altura used on some of their cycle gear. I managed to break one of the hooks and was delighted to find them. No stitching was required!
The only comment I would make is that the manufacturer’s diagram is slightly confusing. It looks like the dimension given is the total width of the hook but in fact it is the width of the strap that fits. My saddle pack required the 20mm size and the pannier straps were 25mm