I saw this in a German museum, a barrel that someone would be locked in as punishment, similar to a pillory. but the victim would be made to walk around town instead

Obviously immediately made me think of the kinky possiblities. being pied etc, but also one with a few holes at the top would mean you could pour things down inside too. whilst leaving whoever was inside unable to do much the hands would be free for masturbation. could be made as the germans did or with a sealed base or maybe even out of clear plastic.

has anyone seen such a thing made already?

Hi. I can't see the picture. Can you repost please?

I love inflatable props and equipment

There a few companies that make inflatable bondage balls, straitjackets etc Rubbereva,Latexcatfish.com demak.com are a few I can think of, but they tend to be expensive

Another option might be inflatable zorb bounce balls
here's one from amazon for $125 made by Happybuy

A collapsible barrel bath might be worth looking into- lightweight and portable enough to be discreet and fairly cheap too. Should be a good start for prototyping and initial testing.

Actual wooden barrels are pretty hard and heavy, you can get old wine cask/barrels pretty easily and they'll be watertight.