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I think the real-world (as opposed to Warhammer) name for those things is Pot de fer ("Iron pot", rather than "fire pot"). You might find somebody who makes one using that as your search term.
Found one here:https://www.fredericus-rex.eu/en/Scale-1-72-Figures-Buildings-Plants-Flag-s/Green-Line-Siege-Equipment-1-72/Green-Line-GL-BEL-008.html?XTCsid=g81fjuvmtsdd040u18e5tuoq62It's 1/72nd so may be a bit small of you're looking for 25/28mm
I'm starting to think you might be right. Oh well, worth a shot. Thanks for the suggestions, all.