THere are tons of ways to do this.

-Good velcro can hold down the box, but doesnt do much for security.

-You can use L brackets and screw them down into the floorboard. Drawbacks are that you have to drill holes into your box, and also the floorboard is thin and weak. You can fix this by replacing the floorboard with some .5" plywood or something. Use the original floorboard as a template.

-A very secure way to do this is to bolt the box from the inside into your trunk. Take the subwoofer out, and put a bolt on the inside of the box into the floorboard or chassis of the car. This way is more difficult, but it will be a a pain for a theif to steal...or you if you ever need to take it out.