In this video you can see how you can create extra storage at the port.This storage can be used to store stuff for the boxes for delivery and stuff for the sand quarry.
IMPORTANT NOTICE. From version 1.18.3 they seem to reset this trick.So, if you had done it before the version 1.18.3, your stuff will be gone after you update to 1.18.3 .If you do it again it works, but i don't know if they will reset it again.