While I like FItbit, I lost 3 of them, at $100 a pop, i switched to the mi mband 1s at $15, it has a heart rate sensor and battery for 30+ days, but it does not have the cool magnetic clasp strap. The Mi band 2 did but it was a little bit bigger. So I did what anyone with a 3d printer does, I modeled up a clam shell for it to make it fit. I used Openscad to model this.
.4 mm nozzle
.1 mm layer height
heated bed at 60 for 1st layer 50 there after
25% fan speed