We just bulit a 4 ft by 3 ft box 2 ft high. Put a bottom in and lined that and the sides with plastic. (then covered with blankets of course) We also put a small door on one side so Mama dog could easily get in and out. Then when the pups got big enough and Mama felt better we shut the door and locked it, to help keep the little guys in. The box was bigger than Mama and babies needed, but comfort is key for us.
I tried to find a pic, but all I have is close ups, so you can't really see the box itself. With a little adjustment, the box is now home to Guido, our chameleon!
Anna