I found a cap that will fit the old cans with 1-1/2" threads today. Tried a cap off a one litre plastic bottle of Diggers Mineral Turps. It fit like a glove. Despite being plastic, these are fairly robust and are lined with some kind of chemical-resistant plastic. This liner forms a thick seal inside which is set apart from the cap itself. They look nothing like the original metal caps of course, but are a suitable replacement.
