OP, zip ties and then duct tape.
When I have had to use hard rifle cases I will lock them, zip tie them, and then a couple wraps of duct tape around both ends.
Just got back from taking the Glock Operator course in Smyrna this week and I traveled with three pistols in a large .mil Pelican case (gun bag, belt, and 47.5 pounds of assorted gear in there) and I was amazed at the level of stupidity I had to deal with at the airport. And from SWA folks who normally are well trained.
One genius insisted I had to use one lock on the pistol case inside the larger hard case, and the other lock could go on the hard case. Another idiot concurred and it took a supervisor to finally get there and take 5 seconds to say "no, that's a hard case, locks go on the outside." After all was done and my other checked bag was gone I let that guy have it...he should have stuck with his first career at Burger King. If he had been new, that's one thing...had been doing the job for 18 months.
Duct tape.