What about a good old fashioned nylon thread looped around a ceramic resistor?
This has the advantage of working 100% once fired if a thyristor is used.
Also an interesting idea to add to the balloon is a high altitude UV radiation monitor made from a clear epoxy encased Bluray burner diode (LED'd ones work fine) connected to a small op-amp and voltage to frequency converter such as a 555.
These are pretty accurate and sensitive to wavelengths between 395 and 375nm (UV-A) so could be used as a primitive ozone depletion sensor.
Could be attached to either the primary or secondary payload and useful science done with it as long as it has been temperature/intensity calibrated on the ground.
Also worth considering, a VCSEL based (0.4mA) atmospheric dust sensor.
Adding solar cells to boost the onboard power might be feasible as the wafer type aren't that heavy and provide about 150mA at 1V per 20cm on a side piece.
Adding a WiFi tracker would also be useful in case the main transmitter breaks.
Use silver demister paint sprayed onto a prepared pizza base :-) use clingfilm held on the surface with PVA glue, or use thin copper tape.
This can double as a 10.250 GHz antenna as the wavelengths are compatible with both systems.