Will this be a scratch build project? If you want to convert a plastic Chinese Junk, then I would suggest trying to find a Mega Bloks "Empress Junk" from the Pirates of the Carribean line. That one is a good size for 28mm. I think there is a cannon or two in that kit, although the cannons are grossly oversized for 28mm, at least for shipboard use. Those could be put ashore in a Chinese fortress.