Here's something I used in the whelping box that worked really well - a seed-starting mat. It heats to 95 degrees (lower than a heating pad) and stays at that temp. During the first week and a half, the pups stayed on the side of the box where this mat was under the pad. A few days ago, I noticed they were moving to the other side of the box, so I turned it off. After I turned it off, they spread out over the whole box, so it must have been too warm for them.
I learned about this at a puppy seminar I went to. I found it on Amazon, and in my usual "more is more" mentality, I bought the largest size, big enough for 4 seed flats. (If you look at pictures I have posted previously, you see the excess mat flap above the pig rail.) I think the 20 x 20 size is perfect. I also bought the thermostat, but as it turned out, I just kept it at the 95 degree temp, so really all it provided me was reassurance that it was warm when I could look down at the thermostat on the floor and see the digital readout.
Also, if you don't have a supply of bed pads (the ones that are used in hospitals and nursing homes) - GET SOME! They are great for so many uses, but especially for dog bedding. They are often sold in bundles at dog shows.