I looked at these last winter. My biggest issues with the Rotopax Storage Pack are no waterproof seal and the inefficient use of interior space compared to the overall footprint. It was made to fit as a modular component of a multi-pack setup like shown below, at the expense of interior space.
Rotopax website says it'll hold "about 2 gallons worth of stuff", yet the exterior dimensions indicate it would hold over 3 gallons if it didn't have all of the mounting holes and handles and interlocking components etc.
I'd rather spend the same money for a Pelican 1400 case and get more storage capacity, more efficient use of internal space, smaller footprint on the rack, and 100% waterproof seal.