Seems to me most were .41 Rimfire, so anything that duplicates or closely simulates those ballistics would be a great place to start.

In a centrefire, that would mean something from .380 Revolver to .450 Bulldog or .450 Adams I guess. No problem shortening existing modern revolver cases to fit.

Easy to chamber up and load down to your comfort level!

Unfortunately my unashamed love of the historically correct stops well short of the obsolete rimfire cartridges, so if it was me getting something made new, I would order a single-barrel Colt 2nd Model replica chambered for a shortened .41 Colt cartridge, loaded to .41 Rimfire ballistics or thereabouts.

Remember too that the correct spelling for Henry Deringer's little gems from Philadelphia doesn't have the "double R": the American arms industry corrupted the spelling when the genre was widely copied sometime around the introduction of breech-loading.