I am calling this project 'done' despite a few glitches. I had a need for a stir plate and also a PWM controller w/SSR for an electric heater (electric smoker for now, HLT down down the road)
Inside the box is a hand-built power supply and 555 timer circuit providing the PWM signal. The selector switch has two purposes:
1) Select the output: When the box is in stir plate mode the SSR will not be active and vice versa.
2) Select the cycle time: The switch will select which capacitors the 555 circuit will use which allows for longer hold times so not to burn out the SSR.
The adjustment knob controls the duty cycle (?), going from near 100% ON to somewhere around 50% ON. The knob provides the same ratio in both modes.
Video shows the box in action w/ a 7/8" stir bar and lamp.