This has been suggested before but bears repeating. Every ship in game should come with a standard 4 tons of cargo space without any type of module, a trunk if you will. This is highly relevant as you can't even accept many missions without these 4 tons of empty space. That is all.
I proposed a simple option to refuse cargo rewards, but this would do too (although it seems a bit inappropriate in the case of small ships).
One thing is clear: commodity rewards need to be implemented better.
Currently commodity rewards stifle gameplay, especially for cmdrs in small combat ships.
People in small ships, up to the Vulture, can't afford to waste space for cargo.
For the sake of gameplay the current commodity reward mechanic has to change one way or the other!