I mentioned our Checkered Giant Serena a couple spaces up about Cardboard Castles--she weighed 19 pounds, 7 ounces, so the cardboard can be constructed to hold a fairly large bunny. The box I used as a starter for the whole thing came with 75 pounds of hay and I used one that contained a dryer drum that was double walled. If they are packed in tight they are very sturdy and layering adds to the strength too. They also like to play with and in a single box too--Ted, an 11 pound lop would take a virgin box and within less than ten minutes would have a hole big to get thru and in twenty minutes he'd have a run through. Ted also loved brown shopping bags and getting inside our cabinets and drawers.