The triac will not usually need a heat sink as the fan current, and triac duty cycle, are generally low enough for free air cooling (but do you own calculations!). With my test fan, the rotation ...

Simple Fan Speed Control Here is a simple mod to allow the speed of any 12V fan to be varied. ... Surround the area with a layer of insulation tape for added protection against short circuits.

PWM or Pulse Width Modulation has generally been regarded as too complicated for PC fan speed control compared against using rheostats or linear voltage regulators such as the LM317.