I like the idea, but I agree with @TargasBR you will still need to be able to shoot it and if you would make it immovable it would be kind of obvious. It just becomes a single or even a triple stack keg; that is immune to damage, has no fuse just a switch in the form of a lid ... so suicide bomb. That you cannot sell, but you aren't planning on selling it anyway.
One of the other smart uses is that as a boarder carrying this all you need to do is get on board, make sure you take out enough of the crew with you and the only counter play is to kill you before you open it.
Keep in mind all they need to do is drop it and open it. If you pick it up after they drop it, they can still open it... while you are holding it. The whole down side of kegs is that you cannot sprint and well a single shot can make you explode. So even if they get on board you make enough distance, shoot the keg and live so you repair in the worst case as a final defense. This wouldn't be possible and making it so strong. Also 3 kegs might not puncture that many more holes, but pack a hell of a punch in killing power, even with one if you should be able to do damage far easier than with a normal keg.
Additionally you make it harder to avoid being boarded on top of this, making it even stronger in comparison. As killing keggers before they get on board if you see them coming is one shot and a pretty big target. One of the main defenses versus kegs is just shoot it before they get near. While a trap chest holder you still need to hit the pirate 2 times, making it easier to actually pull it off. Keep in mind the chest holder isn't expected to survive either so taking a single hit is not the end of the world, they are on a suicide mission.