I have a wicker kennel that I line with washable hospital type pads (invaluable for whelping, potty training, etc. - a must in my opinion). I attend births so that happens on the pads, but then mom and babies relax in the kennel with door open. When the puppies are able to come out on their own I then surround the kennel with a baby play yard fence and add a pad for potty training and later a shallow bowl of water and dry food.
My must haves are lots of washable whelping pads, thermometer, ky, hemostats, Doppler or at least a good stethoscope, nutridrops, breeders edge calcium plus (calsorb is another brand but it makes Mom's vomit - either is only given after the birth of a puppy), nasal aspirator or even better a delee mucuos trap, warming pad or a tube shape filled with beans or rice to warm in microwave that you can form in a circle and make a little cozy nest for pups to rest and keep warm while mom is actively having another (then back with mom because sucking helps stimulate contractions), a scale with bowl to weigh puppies.
Here is a link to my kennel, but a regular one with a quilt over would work too. I know a true whelping box has a bar to keep puppies from being squished between mom and sides, but have never felt the need, but I am home all days with mine and check on them frequently. I did use a real whelping box with my Great Danes though.
Mr. Herzher's Original Wicker Dark Brown Pet Residence, X-Large