I trade in open and I know a lot of people who do to. You need to sacrifice some cargo space for defenses. Remember that escape is the goal after an interdiction, not fighting back. A trade ship can't and shouldn't be able to compete with an fighter ship in combat. IRL a cargo airplane has no chance against a fighter airplane too.
My trade Cutter can carry 640t of cargo and until now I always escaped interdictions. Build: Trade Cutter
My trade Cutter can carry 640t of cargo and until now I always escaped interdictions. Build: Trade Cutter