[freebeard]

15% is not bad if there's obvious room for improvement.

If you flatten the angle of the undertray, it will approach the departure angle. Could the it run above the traverse bar, with an airfoil section added to it?

If the plan is a flat bulkhead at the truncation, there exist boxed cavities (my favorite example it the Cobra Daytona Coupe). Even if the gain is marginal a concave surface would allow lightening the structure. Maybe a little harder to fabricate.