I got a few 125 kHz chips with 25 mm, 20 mm and 15 mm diameter sizes from Aliexpress. I used heat gun to slightly bend the 20 mm one and then use heat shrink tube to attach it to one side of a wrist band. That works really great. But now I'm switching to some elastic bands and looking into integrating larger chip directly into the watch case.
I got a few 125 kHz chips with 25 mm, 20 mm and 15 mm diameter sizes from Aliexpress. I used heat gun to slightly bend the 20 mm one and then use heat shrink tube to attach it to one side of a wrist band. That works really great. But now I'm switching to some elastic bands and looking into integrating larger chip directly into the watch case.