I got to thinking and a bale of straw isn't a bad idea.
My first thought was a flag on a rope, but wind can make it blow up and become less visible. So it needs to have weight in order to hang down and be visible at the right height. It needs to be disposable and cheap. And it can't damage boats.
So a bale of straw is not a bad idea even in 2023.
https://www.legislation.gov.uk/ukpga/1831/22/pdfs/ukpga_1831...
I think its one of those cause and effect things.
You have a horse-drawn taxi. You wish to feed your horse at some point during the day.
The law says you can only do so with either (a) your own hay or (b) hay or corn held in your hands.
So the restrictions of the law push you in the obvious direction of keeping a stack of hay on your carriage.