You're right, there's a false floor over that so you can throw stuff in the trunk.
I had made a 1" false floor out of masonite pegboard laminated to a sheet of styrofoam. Covered it in carpet underpadding underneath and upholstered the top. Cut two reliefs for the drivers in the foam. Then I ran the stock carpet on top of it all, and no one was the wiser.
The false floor weighs about 2 lbs, and it's very strong, you can stand on it. I've made the exact same type in my GTI now.
Here's pic of the underside.