At the halfway point you get 7% more lift but still need to withstand ~50kPa of external pressure. I can't see how you'll ever build a rigid structure that can

At the halfway point you get 7% more lift but still need to withstand ~50kPa of external pressure. I can't see how you'll ever build a rigid structure that can withstand 5 tonnes per square meter and only weighs 7% more than a gas bag which operates only under a tiny fraction of that force and only under tension.