You can get rack mountable fire suppressors, but they are not cheap. Easiest would be to use canister of CO2 with temperature sensitive release - either temperature to pressure converter acting valve or simply melting tube (not that good on long term).
Heat is an issue, I'm blowing (then venting) the majority of that, but I'm curious if anyone else has installed any kind of standalone fire suppression system. Clearly installing an alarm above the machines is easy, but with these machines running unattended for days (and on a few occasions a year, weeks), I would feel better if there was an easy way to set up a suppression system above the machines.