I don't know about those, but I could suggest an infrared light. (heat lamp) We have two in our barn - one in the actually kidding stall and one in the main area. The babies spend a good deal of time under them warming up before heading out to romp some more. You can even purchase larger ones made just like long fluorecents to heat a larger area - those are miracles for horse stalls.
I have seen them before too! On the internet some where. The cut a small door in the front then put the heat lamp in there somehow. might have cut a hole big enough to sting the chain and cord through it. ??? Hope I was some help!
I have made sweaters out of sweatshirt sleeves and they work really well and warming boxes are easy to make. Just use the open end and cut a hole to place the heat lamp on top with the lamp in the hole and it keeps the babies warm and toasty...I peospnally just use a heat lamp out i my kidding pen. I have made 1 out of a 35 gallow water tank and it has the hole in the top If I dont want to use it for heat then I just put the screw on top back on and the older kids sleep in it as its draft free. All I basically had to do is cut the end out of the water tank. I did this as I did use the water tank and figured it would work to keep my bottle babies toasty. It worked!