To give my D8 a chance of warming up as I flit between our houses I've fitted one of these, which I got off ebay. It took about 20 minutes including jacking the car up:

The heater comes with instrukshuns, a tube of high-temperature silicon sealer (the remainder of which looks quite useful), an applicator that doesn't seem to have any purpose whatsoever and about 2 meters of flat cable. All you do is clean the sump, slap it on, seal around the edge with the silicon, run the cable somewhere useful and whallop the connector of your choice on the end.

Testing, testing... The pad gets bluddy hot straight away, after an hour the surrounding area of the sump is too hot to hold your hand on for long, however, the other end of the sump is just warm. Maybe I should have stuffed it in the middle but I was worried about the cable, seeing as I've got no undertray or owt.
Btw, all I've put on it is a fairly normal mains plug (for here), it's a bit beefier than the usual and I've sealed it up a bit. However, if anyone's got a better idea I'd be happy to hear about it.