Many years ago, I tried carpet as a substrate. Very briefly... basically until the first time it got peed on, and I realized that the pee spreads all over the place just like it would with paper towels, only the carpet is then really messy and difficult to clean. At the time I was of hoping to find something with the convenience of paper towels, but re-usable. I think I even used a sheet at one point. But eventually I decided that while it may be more environmentally sensitive to use something you can wash and re-use, it's much easier and involves you in less laundering of snake pee to just use something disposable.