Well I wanted to attach it in a way that would make it easy to take out for luggage, etc so all I did was attach some cheapo picture hanging hardware to the box that allowed it to slide under the trunk lininer floor. So all I need to do to remove it is just lift up the floor, and slide out the box towards the front of the car. The clasps are tight enough, and I put them so that the box doesn't side from side to side at all, even under hard driving.

It's worked great for 2+ years.

Hard to describe, so I took a couple pics. I labeled them cause the box kinda blends right in haha. I don't know how similiar your 5th gen trunk is to mine.