My wife has a bike buoy on her dressed up Raleigh Twenty. It swivels and mounts on the handlebar. Insert can be replaced / removed for a cup w/ handle. Funky cool and works well.
I acquired it probably 25 years ago but just the other day noticed a similar type marketed by Bell (I think?) or perhaps it was Schwinn. Anyhow, it was in the bike section at a Meijer store.