So I had two basic options as to how to lay out the internals of the watch:

Stacked or adjacent.

A stacked layout will provide a skinny appearance but will be taller and potentially more annoying.

An adjacent layout will be wide and flat which could be equally annoying.

After playing with some cardboard models etc I decided that it would be best to stack the design.

watch structure

This is the design I settled on. I reckon I can get it down to around 7mm thick which could be ok.

The battery will be mounted on the underside of the wrist with wires coming through the straps. (Probably won’t work).