Or do what I did, Make the box so big that its nearly impossible to removed from the vehicle without a trained contortionist.
A chain would work fine though, provide you have it mounted on the box that will not break when they try to yank it off.
Also keep in mind, that if the theft is deemed impossible by the would-be thief; rather than grab and go, they will get angry and break as much as possible on the way out. I have seen this happen in person.
Two guys tried to jack a friend of mines subs and amp but couldn't get the crap out of his trunk.
He had two 15" Directed audios and a directed 2500 amp. We walked out of the store to find the two guys messing around in the trunk which they had pryed right open, and we started runnign at them and just like that, one of them took his tire iron and did a stab, stab, smash.
Shoved it through the cones of the 2 subs and then into the rear window.
Just one 10" should be pretty easy to conceal or restrain. A tiedown strap with a padlock on it will also do the trick for a less noticable and gaudy restraint. Or BOLT it straight to the trunk bed, and use torx bolts to secure it same with screws on the box and amp. At least make it take a LONG time if its gonna be attempted.
Time is a thief's greatest enemy.