I just checked and I’m honestly surprised. There are no suggestions about giving cargo an outline like what we have with camps. Probably because it’d be kinda annoying grabbing a piece of cargo and moving it around with a giant green block on the ground. However, I have a bit of a counterpoint…
Just make it show when on the ship. This’d apply to both cargo and sealed chests, and it could have a toggle. It should be pretty easy to implement considering Vetex already did it with camps, and with the option for a toggle it shouldn’t be that intrusive. Alternatively, there could be an opacity setting so you can set how translucent you want the outline to be. Would you like your outline to be mostly see through or look just like the camp outlines?
Of course the best thing to do if at all possible would be to enable shift lock. However, I have read some speculation on there being potential reasons on why it can’t be added. Thus I made this as something that shouldn’t necessarily conflict with anything as it would just be an indication of where the cargo will be placed and whether or not you can place it.