For extra security you can run screws or bolts through the inside of the box to the trunk floor if you remove the sub. This will make theft take a very long time. Otherwise the easy and fast route would be Mr_Kebo's suggestion.
Reguardless which method you use make sure you check and make sure you don't run screws or bolts into anything you shouldn't; like gas lines, the fuel tank itself, etc.. Just want to keep it safe.