I don't know of any way to customize the clock like that. But anyway you would have to get used to being attentive to the time you have left, since in a real tournament you would not have any warning.
Multiple low time warnings?
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Dec 29, 2020
0
#3
I'm not practicing for tournament play. I guess I could write a Greasemonkey script or something to implement this feature for myself.
Furums
Thanks anyway for your answers
A member here at c.c by the name of yashkanrish says somewhere in forums said he developed a small program for rapid 10 mn.games. it (supposedly)warns with a pop up at 3min. Left then a sound alarm each min after that with the last warning at 20 seconds. Printed the address and 7steps to install. As, htts://github.com/yashkanrish/chess.com-low-time, step 1downlode 2 open chrome go to, ... ah hell, I can't remember the rest of it. It was hard for me to find. ( being 61 this month, and absolutely smart ph. ,tablet, and computer illiterate ) . . .l'll find it ,I believe in forums: help and support.
3 days ago
0
#8
I just created a free chrome extension for low time alerts.
It has:
- Custom low time notifications with a few different sounds
- Optional 5 second countdown
- Optional screen flash if you're hard of hearing
You can find it here: https://chromewebstore.google.com/detail/chesscom-low-time-alerts/ndfkkhkkmekhknheeoecnkogaodjecmd
Hopefully this is helpful for you 
Is there a way, or could it be implemented, to give multiple low time warnings? Sometimes the low time warning will sound, but a minute or two later, you forget you're under time pressure. It would be nice if you could customize multiple low time warnings.