Any disc or 3D object you can put in the sea zone would suffice!
I'm leery about adding a Port to Arryn. Would probably just mislead the Arryn player into building ships in Port, and invite Stark to take over the adjoining castle and convert the ships, gaining even more strength. Think you'd want to build land units as Arryn either way.
You could always copy + paste a port from the PBF game boards in the PBF forums.
If the Stark player can destroy the Eyrie and take the ships, he deserves them. The Eyrie is nigh impossible for Stark to take except under the most ideal circumstances.
I do agree that Arryn should not have a port, though. Part of the fun of FfC is that everyone has unbelievable tension, especially when playing House cards. If Arryn could build a ship in port to support his boat(s) in the Narrow Sea, Stark would have a very difficult time ever taking the Narrow Sea.
But with the current setup, Arryn generally can hold his sea by playing a high card (maybe his highest). Stark knows this so he might play a low card to counter. But Arryn knows this so he may play a low card trying to predict a punt by Stark. And it goes on and one. If Arryn ever gets to the point where playing Lothor Brune would guarantee a victory, the tension is completely lost.
I prefer the game as is, so decision such as..."Do I play Bronze Yohn and win against anything or do I play Littlefinger and beat everything but Ned with the solace that if he does play Ned I get 8 tokens or do I think he's just going to use Rodrik and play Nestor Royce?" Stark also goes through these options as well. It is excellent, and IMO, the way the game is meant to be played.