I happened to have an old Casio G-Shock watch. I guess any rigid watch with somewhat flexible armband will work.
Just stretch it a bit and wrap it around the front brake fluid reservoir (see picture 1). No tools or customization necessary! Fits right on. Interestingly enough, on the bottom side of the stock reservoir there's a little notch. It fits perfectly into one of the armband holes (picture 2, circled).
The watch is very secure there and super easy to read. As a bonus I can easily press the night light button (even with my glove on!) when it's dark outside.
How's that for an idea?