The player can't, at this time, dig a cave. One may dig a trench and cover it with structures but that is a lot of work. Haven't tried a Burial Chamber or a Troll Cave. The Chamber seems awfully cramped for a portal. A cleared out Troll Cave wold have to be close to the shore to be worthwhile.

I would build a structure close to the shore just large enough to hold a portal and a chest or two. Trolls won't go after a portal if they can't see it. Load the ore into the chest or chests until you are ready to sail after it.