This app was mentioned in 1 comment, with an average of 2.00 upvotes
Sounds like you're already on-board with the iPhone timer, but if you have Android, this simple app does a good job. I have large Android tablet that I won't need during the draft, so I'm going to use it for our timer.
<https://play.google.com/store/apps/details?id=com.brainfreezesoftware.drafttimer&hl=en>