
I use puppy pen panels (one with a door at the front) on a washable base (I ahve a 3 foot square one soem old puppy panels fitted in, but for a larger pen have used a large offcut of cushion floor vinyl)), and line the sides with cardboard to about 18 inches.
I use a panel across the top to keep it rigid and also to be able to use a throw over the whole to make a cosy cave.
I don't use pig rails as have seen pups hung up on them, and the only pup I ever lost to crushing was in the middle of the box after an exhausting labour for Mum and me where we both fell asleep.