Something like this heated pet mat (101F - 105F)?Don't know whether this would work or not, but perhaps it will get someone's creative juices going:
As I mentioned to Sally a couple days ago, when I was breeding bird dogs we always tried for winter litters. One of the dog supply houses started marketing eletric "nest warmers" (for lack of the proper name...been a lot of years since those days). Anyhow, it was a shallow pan, similar to a wok, that was set into a hole in the floor of the dog box. The idea, obviously, was for the dog to whelp her pups in the heated pan. The power cord was routed outside the box and (in my case, anyway) plugged into a heavy duty exterior extension cord.
I don't know if these, or something similar, are still made, or if they are/were available in smaller sizes, but they may be worth checking out for those in colder climates.
http://www.walmart.com/ip/14660242?...73907192&wl4=&wl5=pla&wl6=81466133192&veh=sem