The insert is a 1 7/16" mini insert. Use a 1 3/8" forstner bit to cut the hole for the insert. Just cut the hole deep enough for the clock to fit flush.
I have a couple days off work this Thursday and Friday. I hope to spend several hours in the workshop and I'm looking forward to every minute. We are coming to the time of year here in the US when woodworkers start spending more time in their shops. It is also a good time to think about shop safety. The next time you go into your shop take a few minutes to think about what you can do to make your shop safer. This is a great hobby but it can be dangerous. Don't let a careless moment turn into a lifetime of regret. Be safe.