Ford superduty diesel trucks come from the factory with a block heater. It basically screws into the water jacket on the block. It draws around 1000 watts. Ford/International claims you only need it on for 3 hours to pre-heat the block, and thats from temperatures below -10 degrees.
You do the math. 10 cents per kilowatt hour (and 1000 watts is a kilowatt) means your eating a buck a day if you left it plugged in all night (say 7pm when you remember to plug it back in, until 5am when you go to drive it away again). So a heavy duty outdoor timer is the way to go. Make sure your extension cords can handle the long term heavy draw, and that the cords going into the block heater itself are in good shape. I've seen lots of pictures of engine fires from faulty block heater cords.
If I had to aftermarket one in, I would try and find one that fits into a freeze plug location, followed by the radiator hose insert style, followed by the oil pan stick on style in that order.
All that said, for a few bucks more, I'd just get a remote start/alarm system. No cords to remember to plug/unplug! Oh the damage I've seen from driving away with the cord still plugged in!!!