Roof top box

[Ziegelsteinfaust]

I am looking at a roof top box for my summer trip to increase lockable storage for my rcsb truck. I will make a rack over the bed for it to mount.

Well the largest Ilsize I found was 18 cu foot, but no matter the size. The weight limit is 110 pounds.

Is this due to the roof rack system a car has?

Or because of the plastic floor the unit is made of?

For reference one of the items I like.

https://www.amazon.com/JEGS-Performa...2&gad_source=1