How to built a water-well for garden decoration?

This is for my mom and it helps me distress from seeing clients. It is one of those old types where you let down a pail and pull it up with water in the pail (old western movies). So I builded the surrounding structure but how can I build a circular roof--like those found on a carousel? How do I get the circular-look for the roof?

Thanks up front and my mom thanks you too. :)