Maybe a pad heater mounted on a say 1/8 inch thick aluminum plate which in turn is RTV'd to the bottom of the ribbed pan casting. Some sort of foam/shiny insulation in the bottom side of the whole mess would be very desirable to minimize radiant heat loss if it didn't project down too far & get into the road crap. Use lots of RTV (lay it down down in separate lines for each rib) & try to get attachment to each fin since RTV really isn't a very good heat conductor. This would probably work for say 150 watts or so.
Yes - definitely synthetic oil but you already are doing that.