This is an enclosure for an off-the-shelf motor speed controller board. Available at: http://www.amazon.com/gp/product/B007TH4EN6
Make sure you get the 12V-40V-10A-400W, not one of the other "color" options.

I'm using it to slow down my ABS fume filter fan so it runs quieter. Works great.

I printed at 100.5% scale for thermal shrink with ABS, 30% linear infill, two shells.

Some small screws are required to fasten the circuit board and lid. The screws strip out easily when printed at 2 shells -- be gentle.

ELECTRICAL WARNING: this PWM board has +12v on one of the two smaller heat sinks. Do not let it touch the grounded heat sink next to it.