Slightly buggy, the counter appears to reset every Thursday with the weeklies. The only strongboxes that count are the underground ones from finding hidden Shield bunkers. I spammed the mission "Stark Realities" over and over with a flying hero, went straight to the hidden vault (it's always in the same spot), grabbed the Strongbox, returned to Quinjet, rinse and repeat. Takes 2-3 hours but is the easiest way to get it.