Small Hydrogen Tank (1x2); Window Blast door; Longer and/or Faster Blast door; Medium Cargo
Small Hydrogen tank would be the same size as o2 tank. Also a Hydrogen tank that is the same size as small ship o2 tank would be nice as well, with it being longer.
Window blast door would save oxygen when a window is about to bust.
A longer and faster blast door would be more helpful in tight situations, and a larger area means a bigger cover for ships.
Medium Cargo containers are absent from large ships - they would be 2x2.