You would probably get good results just making a black hard slip cover with a hole at the top (bee and air escape) and one in front at the bottom for ventilation and clensing flights. Best would be one made of Lexan and clear. The ideal one would be stiff so it stands up and off of the hive body, thus creating an airpocket of space around the hive to get a convection type heating going.
Or on your bottom board you could place a reptile heater pad... They are water and dirt proof so long as they are not punctured as reptiles can poop on them in their cages, but you would need a power source.
I would worry about it getting too warm, so a thermostat of some kind should be installed between the cover and top to shut it off if it gets above 50.
I will not have to worry about it getting to warm here till march. I just wish I would have got a couple units before I wrapped my hive up so I could check out what is going on in one of my hives this winter.