And as such; nope. None of the cargo ships we have currently are "cargo" ships, more like couriers or something. Unless you're mounting the cargo containers outside the vessel, they aren't freighter ships. Way too small. For comparison, this is about the size of Error's cargo hold;
Daynel's ship is about 80 of these.
Vinyl's ship is about 58 of these.
CvN's ship is about 102 of these.
For extra comparison, our container ships today are 300 meters long or bigger. The largest ship is just under 400 meters long, and carries 15,000 of these containers.
So unless you're not transporting a lot of stuff (or transport stuff on the outside) then these aren't freighter ships.
No, shadowcat, you cannot be a cube/sphere. I'll explain why.
Ships are generally long and thin for a reason; how much surface area you present to the enemy compared to how much firepower you can bring to bear. A thin profile allows a maximal ration of this; you can shoot most of your stuff at them while providing a small target to shoot at.
A cube or sphere? Sure, it may be the best shape for space, but no matter what side you're shooting from, half your firepower is unusable and you always provide the same target. It's also ugly as all hell, so no.