About 2 weeks ago, I ordered a 50w and 150w heating pad for the transmission and oil pan respectively. I'd have gotten bigger ones, but a 250w was twice as expensive, and the 500w was too big for the pan.
Kat's 24050 50 Watt 2"x 5" Universal Hot Pad Heater
Kat's 24150 150 Watt 4"x 5" Universal Hot Pad Heater
Permatex 81160 High-Temp Red RTV Silicone Gasket, 3 oz.
They come with a sticky surface, but the instructions recommend that gasket material be placed around the edge to seal out moisture and oil.
50w and 250w are probably about right, anyway. On the same outlet I also have a 400w block heater, 12v trickle charger, and the extension cord I pushed through the firewall, which I use for either grid charging or to preheat the cabin with an electric space heater. The space heater has a 750w setting, and when combined with trickle charger, block and pad heaters, total draw is ~1500w.