Mystra;
The DOOR of the cabin can be locked once you are in the Cabin (of course the Ticket master can open it with a key).
There are sturdy racks inside the cabin (NO lockers etc). Lock your suit case and put it on the rack. Your smaller bag (day bag) should be with you, with all that you need for the journey beacause it is very difficult, due to lack of space, to be getting at your suitcase.
If there are 3 of you, then you "could" have the whole cabin to yourselfs. I have shared cabins with 2 - 3 strangers before and always had a good time. Respect the presence of your fellow passenger as they (hopefully) do yours.
Bring a plastic bag for your garbage etc. And as I said before, bring some food/drink. Some food lends itself well to travel, others like a roasted chicken or a thick sandwich with spicy salami do little to "enhance" the ambiance of the cabin.
Bring ear plugs and a small head-lamp too.
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Vamonos pa'Cadiz!