I know the Roverpet Plastic Puppy Rails for Whelping Boxes.
Fits snuggly and securely inside the Whelping Boxes and reduces the risk of harm to any puppy. Available in White.
Fits a 4′ wide x 4′ long Puppy Pen.
Made out of heavy duty industrial strength PVC.
Easy to clean with soap and water.
Firstly: Plastic Pig Rails can reduce the risk of possible suffocation.
Items Included: #44PR
(2) #PR48LB PVC tubular pipe w/ pre drilled holes for legs
(2) #PR48CB PVC tubular pipe for cross bars
(4) PVC support legs