Starting with a generic activity tracker saves a huge amount of time and cost. This is what they look like on the inside. Usually 90%+ of mass market electronic device manufacturing costs are upfront factory equipment and facilities. Raw material and labor cost are usually extremely low. Because of this a $20 activity tracker will often have the exact same components and build quality as a $300 device. 

Carving up activity tracker enclosure so it will fit into 3D printed locket interface. Holes for wires etc.

I modeled an adapter to connect the back half of the activity tracker enclosure to the modified locket. You can 3D print this adapter yourself using files attached to this project.

Use a dremel cut off wheel to grind out back of locket. 3D printed interface will be glued onto the back of the locket joining it with the rest of the device.

Middle piece of activity tracker fits into interface.

Drill holes in activity tracker enclosure cap for temperature sensor and buzzer. Solder wires onto PPG heart rate detector and solder those wires to the temperature sensor. We can add the temperature sensor onto the same I2C data bus by doing this. Makes everything easier. 

De-solder stock vibration motor from generic activity tracker and replace it with a buzzer.

Glue switch into 3D printed interface and wire to main circuit board.

Combine interface and activity tracker enclosure. Score surface of interface which will be glued to locked. # rule of gluing: when in doubt, score.

Epoxy interface to locket and we are all done! (With device build at least). Now we just have to add a photo of a loved one or something else emotionally significant. This will encourage emotional investment in using the device.