I've been using a similar PWM with a Newport 12V 45 lb TM for about 2 years now, with no issues. I put it in a small WP box from Walmart. I omitted the display, but used the switch and control pot. I did bypass the controls in the TM and wired directly to the motor leads.

If you smoked the PWM, you had to have something miswired; likely the input power, because the PWM would work fine with reversed motor leads, just in reversed direction. The input power from the battery cannot be reversed, or connected to the output terminals, without something bad happening (smoke).

A couple folks copied my implementation and theirs also works fine. I built one for a friend and his is still working fine. There is a thread on the GON.com forum search for "speed control".

The module I used looks just like this one. Robot Check I threw out the display and the metal shield. The switch is crappy and has been replaced a few times - it isn't waterproof I guess.