I've made something similar in VoiceAttack. Haven't had a canopy breach since but i've tested it by turning off Lifesupport and it seems to work. In VA as soon as I turn off Lifesupport I say "Lifesupport off" and my AI tells me I have 10 minutes oxygen left (it checks my installed Lifesupport grade). Then it notifies me after I've reached 75%, 50%, 25%, 1 minute, 30 seconds, 10 seconds and then says death is imminent. When I turn Lifesupport back on (or when I enter a station) I say "Lifesupport on" and it stops the countdown.
Now I've integrated it with EDDI and it should start automatically when my canopy is breached. I tested this with the Undocked event and as soon I undocked the countdown started so that seems to work. It stops when I dock. Now I just have to wait for a canopy breach

Maybe I'll try with a cheap Sidewinder.