Concealing 6-inch guns in a cargo raider

Which features matter most when an interwar freighter is secretly designed as an auxiliary cruiser?

It needs range, a useful burst of speed, and cargo spaces arranged to conceal a respectable battery of roughly 6-inch guns.

A useful disguised raider would be a light cargo ship that is unexpectedly fast and has long cruising range. Its holds would be arranged so guns and their mountings could be hidden until required. A roughly 6-inch battery offers useful raider firepower without pretending that a merchant hull can become a battleship.