We need to know more of your product requirements:
- how long is it going to be worn
- what kind of sensors are integrated
- during what activities is this going to be worn
- what is the demographic
What comes to mind is a type of elastic ankle bracelet with a custom designed hooking device that also integrates the electronics
There is no such thing as ‘no locking mechanism’ - since something always stays in place because of a mechanical reason - maybe you mean that the mechanism needs to be integral to the material. Then consider developing a small bandaid-like patch that a gecko dry adhesive sticks to.