Yup, big black binder clips that you can buy at walmart - 6 large or 12 medium sized for like $1 - $1.50. I usually use a couple on every side, and then I'll put something relatively heavy (like a big book) in the center of the lid just for some extra pressure. Some people use bungee cords too, but I prefer the binder clips.
As for why she seems more skittish, if you did just recently move her to the tub, she's probably still getting used to it and might feel more insecure. So everything's going to seem more scary to her until she settles down. Give her a week or two and she will probably be fine
Oh, and yup, depending on how much she's drinking, that's normal. Anything white is pee, and sometimes if they have a big meal they will poop twice instead of once. Doesn't sound like anything to worry about
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