I LOVE it! And, I HAVE a few that I was going to donate, one flannel & one percale. (the percale one has a zipper too) Hmm, thanks!
When I suggested a pillow case, I pictured them slowly moving it when they push from the inside, sorta like a hamster ball, but I like the "duvet"
idea even better! The only thing though, with all that activity, is I'll likely put a sheet of plastic underneath, just in case they get "inspired".
One word of caution: when a snake that is more visually oriented (like our rat snakes) have their view obscured (like in a bag of some kind),
they sometimes get cranky when we go to retrieve them: it's just them being startled & afraid, they may need a moment to realize they do
know us. (I noticed this with rattlesnakes years ago...they really resented being "bagged" presumably because they couldn't see what was
happening. They're pretty smart snakes.)