Inside and Out - Trunk Issues

Alright in my trunk i have a loose floor board that rattles around when i drive over speed bumps, or when i drive fast which is most of the time. So I was wondering if you guys had any tips on how to keep the floor board in place. I was planning on nailing it down, but what would i nail it to? Any help would be appreciated thanks.

You usually don't want to nail down anything in the car ;)

Try to post a pic, this might help to undestand your problem