I use an upside down popup laundry basket for the 5th instars to pupate in. I found out that a square melamine catering tray from some office party fit perfectly to provide a sturdy base... AND a full sized paper towel sheet fits perfectly in the tray.
This morning, when I returned with the freshly paper towel lined tray, leaves, and happily munching cats, one of the J's on the laundry basket bottom (which is the top you understand) was stretched out. My initial reaction was ANNOYANCE - "Aw! I can't move the basket back onto the tray now! This one's about to pupate!" ...then I thought, wait, how do I know that?
I watched, bemused, and sure enough, not even a minute passed when the cat started to flex along its length. I watched the entire transformation until the "Let's Do the Twist!" and the cat skin dropped off.