Take two thermal fuses (like this one from Newark) and wire them in parallel using a little prototyping board. Mount it on the firewall, run a circuit through it, and wire to the G3X. If the circuit ever breaks, then the G3X should show a FIRE warning or similar.

The important bits – the thermal fuse should be one that works around 350 degrees Fahrenheit or so. It’s up to you how many you want, but two are suggested. Wire them in parallel – then if the circuit goes open then you know that both fuses blew and you have a high confidence in the alert you’ll get being true.

An improvement to this circuit might be to put some LEDs or similar into the circuit so that you can preflight it and see both LEDs on showing the fuse is in good shape. You could even be fancy about this and mount those LEDs on your panel with a “test” button to put them into the circuit. In this case, you’d have a longer circuit for each parallel fuse – it would need to contain the LEDs before going back.