I think you'd need some sort of direct anti-wall mechanic. Otherwise people will just build fast walls. It's very hard to design something that can tank multi-vector attacks without being a Borg Cube, but it's pretty easy to make something that can outrun anything because of the way speed only scales with wall thickness, not length, in wall designs.
FTL streamlining wouldn't work, though. A wall with the control room mounted sideways is suddenly a needle in FTL.
Putting a mass squared term in the FTL cost might help, but non-wall ships tend to rely on heavy armor because it's so difficult to multi-layer shield around corners. You can have what are essentially bent walls, but it's basically impossible to shield multi-axis designs so much they don't need lots of heavy armor. I think you'd just get small walls.