You can buy clock innards from many online stores. They are usually quite inexpensive and run off a battery. You can get them in a variety of styles. If you want to add on numbers to your cover you can do that. It's fine, too, to simply leave it hands-only.
To get the clock to fit, you carve out the pages in a shape to hold the clock unit. Add a battery. Now simply stick the unit with the hands through a hole into that mechanism. It takes maybe five minutes at most. The "hardest part" is carving out the space in the pages to hold the main mechanism. A box cutter or Xacto blade works well for this, with adult supervision if necessary.
It's fun to make these in a variety of styles.
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