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I would look for a heavier cable than most of the luggage type security. The cables used for bicycles will need a bigger cutter and a little bit more time to cut through. Either get one with an integrated combo lock on one end like sjms suggested, or get a cable with loops on each end and then get a good standard size combination lock or key lock that will fit through the locking hole on the Pelican case. And if you want some "extra", you can actually put two cables on one Pelican case, because they have two locking holes.
Of course any cable only slows someone down. The first level of protection is out of view, like in a trunk or hidden under a roll out cover in an SUV. If things are in view but covered by clothing or stuff, they are still attractive, and your mention of locking to a seat frame may end up in that situation. If there is a trunk, it is a much better place, and you can still lock the case in to the car back there. Also, no matter how or where you are securing things, make sure you adjust your gear so that you can have things all in place and secure before you arrive at your destination. You do not want to “expose” your gear to view as when you arrive as you select what you need before you leave the car, because you may be observed, and then you are a well known sure thing target. And some form of car alarm may help a little, depending on how many people are around at a given location.