Whoop 4.0 Casio Watch Mount

This model allows you to attach a F-91W Casio Watch ($17 on Amazon) to a Whoop 4.0 fitness monitor.  It uses the existing slide-on strategy that the Whoop battery uses so you can remove the watch when you need to charge the Whoop. 

Installation only requires you to remove the pins on the existing Casio watch band and then use them to hold the watch in place on the plastic mount. Then slide the entire assembly on top of the Whoop. It holds fairly well, but can be dislodged with a solid bump. 

The final result is quite ugly and tall on the wrist, but there might be a use case when you need a simple timepiece on your wrist that will allow you to keep your Whoop active.