I agree that those pad heaters are flimsy. The magnet style are overpriced for their output and a hassle if you have to remove them every time. There are hard elements that can be bolted on though which must be very durable.
The catalogs are here:
Heavy duty catalog link including the tank style and oil pan heaters:
http://www.fuelinjection.net/zerosta..._zerostart.pdf
Main catalog:
Zero Start Product and Applications Guide