555 as slow PWM driver
With tantalum electrolytic capacitors you can choose ones with ultra low leakage @ Vmax equiv to  1M to 10M +.

But it is far better to use a Triac and temp sensor and driver to triac  with hysteresis that controls the drift temp. about any setpoint or use phase control for linear PWM control with some gain. to reduce the error or PID loop for ultra fine control. (&lt;&lt;1'C)