I'm a bit confused. Are you planning on keeping her and the pups outside? If so you don't need a whelping box, as they don't have a roof. You need a good, solid shelter that will protect mom and babies from the elements.
If you're going to have her inside, a child's wading pool makes a good whelping box. It's easy to clean, Mom can get in and out without any problems, but puppies stay in until they're actually walking. You need to cover the entire bottom with several layers of newspaper, then provide blankets to keep Mom and babies warm.
For extra warmth, you can put a heating mat, like one would use for reptiles, under one side. Puppies can crawl to that side and keep warm during the times Mom is out, taking a potty break, or just taking a break from the responsibilities of puppy rearing.
And I know you didn't ask, but I'd recommend you don't place the puppies in new homes until they're at least 8 weeks old, preferably 12 weeks. They need time with their mom to learn how to be a dog, and to learn bite inhibition.
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