One Halon alternative is the rather simple and cheap gas CO2. It is neither toxic nor hard to get: "60 liter" water-carbonation bottles are readily available over the counter to create soda water and contain some 200-300 liters CO2
Alternatives are Argon and Nitrogen, but those don't come in these small canisters and dissipate more quickly, but they also need the chamber to be gas-sealed.
Because non-flammable gases act by smothering the flames, it is vital that rooms in which such a gas firefighting system is installed are not rooms in which people are usually, and in case of a workshop, it might be required to disarm the automatic trigger while the door is opened!
Such a soda water machine also would deliver the most crucial parts of a deployment system: the valve. To activate it, a solenoid can be used - once current flows through the solenoid, the valve is pushed open and the CO2 will shoot out into tubing or piping leading into the build chamber.
The gas canister should under no circumstances be inside the build chamber's heated compartment but somewhat close.
Now we need a way to trigger the solenoid. I propose to use at least some redundancy in construction.
- Manual Mechanical Override. This can be as simple as a lever that bypasses the solenoid and activates the valve manually. Also useful for functionality tests.
- Manual Electric Override. Using a 9V block battery and some sort of latching switch/button. The solenoid is directly powered from the "reserve" battery now.
- Automatic via a microcontroller. Using a small programmable chip, one could use a spare thermosensor inside the chamber to detect temperatures of dangerous levels. As a direct response, it triggers the solenoid to release the gas into the chamber and continue to do so for at least long enough to fully flush the chamber, even as the temperature drops with the injected firefighting agent. It might be best to also trigger an alarm, for example, a repurposed doorbell. If one goes for such a solution, an output of the current chamber temperature could also be integrated, though I strongly discourage from integrating the fire suppression as a part of the printer board!
- Automatic via a thermal switch. More simple in design, a Normally Open Thermostat Switch could be used instead of a whole microcontroller. Such a switch could use the same power source as a manual electric override switch, and indeed, be mounted parallel to the same switch. The downside of a hard-wired thermostat switch is, that it sets a hardwired maximum chamber temperature. It'd be best to choose a switch with a reset-temperature that is as far from the trigger temperature as possible to ensure that the chamber contains as little O2 as possible. The availability of switch types here dictates the voltage of the system.
What about the chamber heat control?
Once the firefighting system engages, the chamber should no longer get access to fresh air. This could possibly be made by having a different solenoid push a shutter in front of the air intakes and/or by de-powering any chamber cooling fans.
A magnetic switch in the door should cut power to the solenoid, ensuring that only the mechanical manual override can fire the system.