Looking for Roof Storage ideas

I have a truck camper but I suppose this topic is not limited to that section. What types of roof storage do you use and is it water tight? How is it fastend to the roof? And if you were to do it again, what would you do different?

I was thinking of a car topper to put up on mine but I'm a little aprehensive about attaching it by screwing it into the roof and if I would actually hit a support beam to hold the unit up there while traveling down the road.

I searched the forums for "roof storage" but it turned up nothing.