How do you carry soap for washing yourself in hand-basins and showers in hostels?

I used to take a part-used bar of soap in a small light plastic soap container. But I lost the container (left in a hostel shower somewhere!) some years ago, and I have not been able to buy another. Some stores sell thick heavy containers that will take a large bar of soap, but these are unsuitable for weight reasons. The YHA adventure shop told me they have never been asked for such a thing. So what do people use nowadays?