I used some heavy-ish cardboard that an unfolding bookcase from OSH (Orchard Supply Store) had come in. I cut off the back, and basically following the instructions from making the corplast tray, I made a--well, cardboard tray, lol, with two-inch sides, held together with coroplast. It's very sturdy, and our calico sometimes likes to go snooze on the end next to the window, lol! I don't have pictures yet...
I'm making a second level, and to fix the problem Skyenoir mentioned, about not being able to reach the pigs on the first floor, I'm going to attatch all (okay, most) the front cubes with twisty ties, so I can undo them and swing the cubes down when I want to get them...
~Cactus