Microsoft in its infinite wis-dumb made Windows updates that made my old timer program cease to work. Thanks again, Windows. But…I found one that works quite well, and it runs full-screen.
Type in the minutes you want it to count backwards from, click “Set”, and click “Start”. Kids are fascinated with the thousandths that are counting down underneath, especially when you tell them that they are changing 1000 times a second.
I added the site to my upper menu bar by dragging the icon to the left of the URL to the menu bar. That works with any site you wish to keep close at hand, and if you do it in Google after signing in, it will be on all of your devices when you sign into them.