The Amazfit Balance smart watch is a nice watch, but the exposed design of the screen glass make it keen to get scratches easily. There are some screen protectors that can be added, mainly two types: Tempered glass and not tempered glass (all the "plastic" stuff, hydrogel and so on). Du to the curved edge of the screen, tempered glass protector is ideal, because it is necessarily smaller than the watch glass (only covers the flat portion). Even with an rounded edge, the thickness of the glass make the edge very visible (reflect light). Pretty unesthetic result...
So, I rather chose a 3D non-glass protector (this one is stated as PMMA materiel) which is commonly sold on Amazon or Aliexpress. It also covers also the flat portion of the watch glass, and to avoid the ugly border, it has a thin black curved edge that comes down to hide the edge of the adhesive.
Whatever the type of protector, one problem remains: It's pretty difficult to achieve perfect centering between the protector and the watch screen.

So, after two failed attempts, I decided to design and print a centering tool that helps also to reduce / avoid the risk of air bubbles.

The tool is optimized for the model of screen protector shown in the screenshot. It has a diameter of 39.2 mm. If you have a protector with a slightly different diameter, you will have to tinker the STL file.

Anycubic

Kobra 3 Combo

No

No

0.2

15

PLA is just fine. Resolution of 0.20 is OK, 0.16 or 0.12 will allow more precise shape of the top curved shape of the applicator which favorizes the center contact of the adhesive with the watch glass.