I think the MC could use a smart way to keep things preserved with a simple way: a mixture of wet and then frozen Lan to keep the ice longer along with a dry clay protection to keep the temperature longer where the products are stored, this could be used to preserve things on a ship or boat.
He should have waited for the ironborn to revels to come back, he would have had the time to get much better ship and a lot more food, he's not needed right now