Under bed storage bins work great; I keep it inside a 4 x 4 xpen. Plenty of room for mom to move to one side if she gets warm on the heating pad, low enough for her to jump out easily, but high enough to keep pups in for about 3-31/2 wks. When they learn to climb out is about the time for a bigger play area anyway.
When you're done using it, clean it out & store your whelping supplies, blankets, and such inside & slide it under a bed.
