Thanks, Frank and bunnies Sonny and Bink! Frank writes, "We bought a carton-snack-house (no idea what it's called), and Sonny and Bink are working on it for weeks already.... And they have a very funny way of coöperating... The boxes are great, carton boxes completely covert with hay and carrot pieces. Keeps them busy for weeks."
Edit: Frank lives in the Netherlands, and says these boxes, made by a German company, are available there. As for the rest of us? There seems to be a market for these, so maybe someone in Europe will seize the opportunity!