Well, I am not exactly sure that a pot that small would be multiple plates - I would think it would have to be the size of a gallon or larger to require that.Cast Iron is definitely much later. Items in cast iron appear in documentary evidence from @ 1420, but are very rare until the last decades of the 15th century. Extant cast iron items, although very rare (some domestic, some military) are in collections, dating to 1450 at earliest. Much too late for what you are looking for.
Both Medieval Design, of Italy, and Matuls, of Poland have small brass or copper pots in their wares, not tinned, but such a thing can usually be done locally. YThey are of a piece, not built up from multiple plates.
Goose Bay Workshops, in the US, does fine work, but is expensive. The only medieval-specific item he makes is a lantern, he has pots as early as the 17th century in his cataolg, but he does entertain custom comissions. His work for eating out of is tinned, and is of very high quality.
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Bob R.