I’ve made a browser source based on the open source jQuery timer library. You can enter the following url as a browser source in OBS:

https://inkydom.tv/timer/timer.html?text=time&minutes=5

Here is an embedded example:

Note, you can change the text and minutes. Minutes can be any positive whole number, and text must be a url-safe string.

You can use the below tool I made to create a ‘url-safe’ string, the weird characters won’t show up in OBS.