If you want to ensure no one can open your luggage, travel with a firearm. It doesn't have to be a real firearm, but if you declare the firearm you are required by the airlines to have a lock on your luggage that can't be opened by TSA.
I'm kidding, of course. There are probably multiple reasons that wouldn't work. But this whole notion that by molesting passengers sufficiently and putting on a big enough show, we can significantly reduce casualties from terrorism is laughable.
At some point there has to be a tradeoff between safety and convenience, or no one in their right mind would drive a car.
More often than not, the TSA will want to hand-inspect firearm-containing baggage (with you present), and I couldn't say what they'd do if they didn't find the firearm in the bag they were breaking their routine to check.
Just buy a flare gun or a starter pistol and declare it if you don't want to or can't use the real thing.