White Safety Rope Net for Kids, Exterior Guardrail Net for Stair Balcony, Knotless Fence Protective Netting, Outdoor Terrace Decoration Grid(Size:1M×2M(3.3FT×6.5FT),Color:White)

How about a horizontal loft/cargo safety net underneath/across the opening? They're relatively inexpensive, don't block the open feel of the loft, and can usually be installed and removed without permanently altering the railing, making them a practical option even in a rental. Something along these lines ... The cats can be daredevils and you can sleep soundly. safely net