I have an ultratherm mat and I just foil taped it to the underside of my cage with no polystyrene.

For the original question, I'm using the Repti-Temp 500R thermostat and it works fine. You can't just dial up a temp, set it, and forget it though. It's more or less a fancy dimmer switch. You adjust the power (temperature) by turning a knob to the left or the right rather than setting it for x degree. Once you have your temp dialed in for how you want it you still have to be mindful of things because unless you have a very steady room temperature your cage temps will fluctuate.