did you have any luck with this? i was also looking into getting a fire extinguisher, automatic of course, didnt have much luck though, most of the small devices are overpriced and dont really seem adequate enough to put out any fires, they probably are though, but i still wouldnt trust them and at the end of the day i would want one to put my mind at ease. The route of industrial style sprinklers might be worth checkin out or even a garden sprinkler connected to mains water supply hooked up to a heat sensor could work but im delving into the realms of fantasy here lol, the plumbing and electrical knowledge would be way beyond me...

Dont laugh but ive suspended 2 big buckets of water attached to string and 2 pulleys that would cause them to empty onto the grow if the string was to burn through which although, admittedly it isa pure bodge im confident it would work.