My understanding was that the 0.5 multiplier to shipping does not halve the effective number of spaces in a vehicle, but that it simply changes the vehicle's shipping volume per space stat.
For example, the areoplane type has a default shipping volume stat of 1dT per space, so an open-frame airplane has a shipping volume of 0.5 dT per space. The grav vehicle type has a shipping volume of 0.5 dT per space, so an open-frame grav vehicle has a shipping volume of 0.25 dT per space.