I have bed sheets (2 sets, I change it every Friday and throw it in the wash). It's mostly to protect my carpet from the carpet-crazy Cinnabun. She really likes it. I wanted to get something like you were considering with the puzzle pads, but in the end I went with sheets cause they were a bit cheaper and softer. Here's a pic of her in her in Xpen before bed time. It's also a 4x4 but I made it rectangular to make it fit the room easier. Where she is laying, I have put down to pillow cases and then a small area rug just in case she gets any bright ideas at night about digging.