I know I said this above, but I will irritate, you should try the tv in a cubboard idea, if you only use a 13" tv it will put off a lot of heat, the black substrate will only really retain heat if it is spotlighted, as in with sun, a hot light bulb or similar source. Aluminum holds heat very well, heats up fast also, I am not talking about foil, but aluminum sheet, you could line a cubboard with aluminum and put a tv in there with the containers, with doors closed, you should be able to maintain an even temp inside the cubboard. Of course you would want glass in the doors so you could see what was going on. A bookcase, china cabinet, a stand up dresser, almost anything can be used to make this area warm. I have a dresser in my bedroom that has 3 drawers on the bottom and a 2 shelf cubboard w/doors on top. We have a 13" tv in there, a vcr and cable box, even with none of them on they radiate heat, someone even mentioned a computer that throws off heat, of course you would not want to use it for this. Also if you are using bulbs, the thing to do is to make sure one is fairly new, so if one burns out the other is still on. Long ago when my boys were little, we lived on a farm, I made an incubator out of a picnic cooler. It was a large one, like you see today, at least 30" length and 18" wide and 20 " or so deep. I drilled holes one in each side by the handles and hooked up the porceline lighting fixtures one on each side. I put a wire shelf in the bottom and used a plexiglass lid instead of the cooler lid. With this set up, water stayed in the very bottom (small amount, not enough to cover bottom) but enough to keep the eggs in moist air, and I put the eggs in there and kept them at 102 F. and with this setup I hatched turkey, geese, ducks in this incubator. If it got to hot I cracked the lid a little. This took up a couple feet of room, but I kept it for 30 years and it always worked. the babies stayed in there till dry then I would move them, this was with 40 watt bulbs, the temp could be adjusted with lower watt bulbs and some air holes drilled in top of plexiglass. That's 2 of the things I would do.
Hush up Yen . ((The idol are to large for a set up like this )). I would need a refrigerator sized cooler to do that! He knows what I mean.
But the cubboard idea would probably work and you could always heat it with a lamp too along with or without the tv, cause the heat as long as the doors stayed closed would heat the whole enclosure.