I don't think most folks put enough thought into their whelping box.  Frankly most look like crap.  Nobody wants to by a top-shelf dog out of a cardboard box or baby pool.  If you are breeding nice dogs, think about the details.  If you sell long-distance, pictures are very important and your whelping box is in most of the puppy pictures.  With a little thought (time, effort and money) you can build something that is attractive AND functional.  Dovie's box set me back over $200 for the materials and probably 10 hours of labor, but it will last ten years or more.

Ray built this one 3' x 4', which I think is a very nice size for a GSP.  Mine is similar, but 4' x 4' and I think maybe too big.  the copper rails look nice and are easy to keep clean.  Wooden ones just seem to get covered in puppy ick.  In addition, you can see if a pup is behind them and easily remove when you don't need them so the pups don't have a step to get out of the box.  Floor is melamine, which is super easy to keep clean, but tough for the pups to walk on, so you have to cover it.  Note the tarp under the box, that'll be handy to keep spills from the carpet.