Just use an incandescent light dimmer switch. I'm sure this has been covered in the past. Make sure the dimmer is capable of handling the wattage of the mantle.
I got a rotary dimmer and an outlet from the hardware store, mounted them in a water tight 4.5" square electrical box and wired them together so that the dimmer controls whatever is plugged into the outlet. A grounded power cord connected to the box via water tight connection completes the package. The white wire connects to the neutral side of the outlet, the black wire connects to the dimmer which in turn connects to the other side of the outlet. The green wire gets attached to the grounding screw inside the box. Screw on a water tight coverplate and pop the dimmer knob on and your ready to roll with a twenty dollar, spill-proof power supply.
You'll want to get a feel for where to set the dial for the temp you want, and make some marks with a permanent marker.