You can buy a bucket with a lid on, but they`re not cheap.
You could get a 25 litre barrel and fill your bucket when you get to work. They don`t cost much and you could get one free, off a wfp user or from a car wash. I used to use 5 litre screenwash tubs though.
My customers used to want to give me water and I would often let them. They thought they were helping and getting clean water on their windows and I was keeping them happy.
Hope that helps you, Lewis Hamilton.