Epoxying a 100 kΩ thermistor to a 909 Ω resistor (because I have a bunch of them) serves as a simple heater tester:
It dissipates 450 mW, raising the temperature enough to let the PWM control kick in, but not enough to get scary hot.
Some heatstink tubing
prevents reduces the likelihood of horrible accidents involving the 20 V motor / heater power supply:
Keeping it under 50 °C seems like a Good Idea:
Not that I have a need to heat anything, but the MOSFETs work!