Bought one once and thought about it and never used it. This is before synthetic oil. Might work okay in synthetic, as it will take higher temps, but doubt it will do much. You'd probably do better with a little electric space heater turned on 15 to 30 minutes ahead of time, particularly if it'll fit underneath.
This... A block heater is much better or if that's too hard to replace. Get a radiator hose heater. Warming the coolant does a much better job of warming the engine. A cold engine cools the fuel air mix that you're trying to heat in a diesel engine. Remember... It's the compressed air that ignites the diesel fuel.