BP's generally dont soak unless they have mites or your humidity is off. just use what ever bowl fits in there nicely and is not tipable.

get a digital guage for you humidity and then if needed you can move bowl as neededto regualt the humidity.

your thinking a little bit to far into this, not that its bad but a decent waterbowl really the only requiremnt. size, shape and look is up to you. just make sure they can drink water from it lol