I start with some preliminary sketches.

There were many more, with very little polish in any of them. They're just to get the mechanicals down and direct the overall thrust of the design.

Watch crystals are quite inexpensive from China, so I picked up a selection of sizes, and while waiting for shipping, I modeled and 3D printed some tests for the three most likely candidates. Shown are 30mm, 34mm, and 38mm, but you really can't tell. The 38mm one seemed to fit best on my wrist, so that's what I went with.

Rolling my sketches into the CAD, I came up with this:

The main body is just a shell with internal threads, and inside there's a stack-up consisting of:

• 1.2mm crystal face
• 37mm OD x 1.5mm o-ring
• Double sided o-ring retainer
• Another o-ring
• 15x3x1mm neodymium magnet
• Faceplate PCB
• Control PCB
• Coin cell battery
• Threaded cap

It's a very tight system. As designed, it doesn't really fit with the battery.