I just built one using the basic dimenstion from senecal Kennels. It has worked great. Ist litter she had 6 and all are doing great. I bought a heated pet mat from a farm supply store and built it in flush with the floor. I used newspaper for bedding and when she had the pups I took it out and just use the heat from the pet mat. Seems to work great. If i do another one, I will insulate it. I think I will have problems if it is really cold out.
Hey guys most whelping boxes I've seen or built have a 2x4 raised up from the floor 3" to 4" around the wall on the inside to help keep the pups from being mashed. I let a friend of mine raise a litter of pups from my female and he fixed that with something I like better. He used 4" PVC connected with 90' angles on all 4 corners forming a rectangle that he placed inside the house supported with PVC Tees that kept it up and off of the floor. This removed any sharp edges as well and looked and worked great.