I don't know enough about how dice drops work to make the perfect template. I know I want it larger than many I've seen, because I want it visible by all the players sitting around the table. I tried to make it look aesthetically pleasing too.
I threw together some public domain icons to get a sense of what it might look like. Gathering images for all the goods will be a challenge, as always, with our century long copyright. But you can see how it might work:
|Livestock, hides, pottery, oil, and glassware.|
Probably should have a space for the name of the port somewhere, now that I think of it. Also, I made twenty spots, but this might benefit from a big simplification to six, especially if the ports are supposed to feel unique with different selections from the 20 basic trade goods.
So the idea is the same as from my gear bartering post: throw a d2 and d4 for each merchant. d2 is what they're offering, d4 what they want. Players with exceptional charisma can shift one of these numbers up or down for each merchant. I'm, guessing that we would need some sense of units. That might be something good to have on the trade goods chart, so oil might be kegs, salt could be coffers, hides bundles of certain weight, etc.
Here is the blank template if you want to fool around with it.