There's nothing impossible about a ship that looks like a giant wall or box; obviously there are no aerodynamic concerns, and whatever structural problems arise can be compensated for by the right supports. IRL there will be other considerations (particularly armor) that will determine the shape of space warships. For a good treatment of that, you can play Children of a Dead Earth, or just read that game's dev's explanation here:
https://childrenofadeadearth.wordpress.com/2016/04/12/why-does-it-look-like-that-part-2/
Long story short, the way of the future is cylinders and cones, more like this (from Children of a Dead Earth):
![image https://i.imgur.com/QcChhU9.jpg](https://i.imgur.com/QcChhU9.jpg)
But you may have noticed that Cosmoteer is not a realistic sort of game, so the spacecraft in it will never really look like any sort of actual ones; making something vaguely sleek or battleship-shaped is only pretending.
That having been said, I can totally get on board with making things that are sleek and/or vaguely battleship-shaped, because they're cooler!
![image https://i.imgur.com/9HX0Tjp.png](https://i.imgur.com/9HX0Tjp.png)
![image https://i.imgur.com/dZPMHIH.png](https://i.imgur.com/dZPMHIH.png)