We don't store it on the hard lid as it fits neatly and easily in the back of the car, on top of a crate with a couple of the click together mats to stop it rubbing or rattles.
We made an aluminium frame to fit the back area of the car and this gives us two side bins to store a multitude of stuff, space for stacker drawers and a large crate and a shelf for extras like chairs etc.
All light weight (4kgs) easy to load when we go away, and easily removable....and now travelled over 20,000 kms without moving. I'm surprised but pleased it's worked so well.