Can you run the TCXO off of a higher voltage? e.g. run a 5V TCXO off a 12V source. Use a resistor in series with the TCXO to drop the excess voltage and generate a bit of heat. If necessary, use a shunt voltage regulator across the TCXO to maintain the 5V supply and maybe generate a bit more heat. Adjust the series resistor to draw whatever current you want and generate whatever amount of heat you want. Wrap everything in insulation to keep it warm.
Ignoring the shunt regulator, dropping 7V @ 3ma will only generate 21 mW of heat, but with good insulation, it might be enough to keep the TCXO above -40C.