
I am someone who doesn't like it too warm in the house.
I create a whelping area using puppy pen panels, with one across the top at the back. I line around the sides to about 18 inches with cardboard using cable ties punched through. the front panel is either a gated or drop fronted one.
I have a medium size breed, and use either a 3 foot square (has a solid base to fit it into) or with a large litter four foot square (use an off-cut of cushion-floor lino under) pen.
Over the top and sides I use an old bedspread so the only open area is the upper part of the front to which I attach a household thermometer (they sell all sorts and sizes in our pound shops).
In the pen I use a heat pad under the vet bed, and keep the temperature in the pen above 20'C. for heating over and above the background heating of my Central Heating which is set at 15 - 18'C normally I stand one of those electric oil filled thermostatically controlled radiators on the window side of the whelping pen.
This I set to maintain the whelping pen at the required temperature, so it often only comes on when the CH is off.
As mine is a coated Nordic breed I find that after a week ti is obvious that 20'c is too warm for the pups and I turn the thermostat down a bit, ans by the time they are 10 days old it's just the heat pad, and background heating. Buy two weeks mine don't want heat in the box unless we have severe weather when I will put it on at night.