I still think a lengthened trimaster based on the 3940 or 5135 would be the best solution.
Okay, Atto, you're right about it's decks being more galleon-like, but such a trimaster could be used for scenes from ca. 1600 till ca. 1800, instead of just the late-18th/early-19th century. I guess they would just have to make a few alterations to the mould and make a stretched-out version.
It would be very costly to create a complete new large trimaster-design, probably be not commercially viable as just some collectors and/or ship-fanatcis would buy it because of it's huge pricetag, and so it would likely end up the way the Mississippi Steamboat did: being a cabinet-display item in the design department's quarters (perhaps they already have a trimaster-prototype display in their offices).