Someone on the M535i SIG mailing list found part # 71 60 1 1 126 613 which is listed as a reinforcement for the factory towing kit and which looked suspiciosly like a rear shock tower brace. I ordered one right away from Maximillian. Two weeks later it arrived. The end plates are made of 2mm thick steel sheet. The cross bar is 20mm square tubing with a 2.5mm wall thickness. The bar is welded to the end plates with only slightly booger-looking weld beads. The whole assembly is painted black. It looks flat enough to fit under the amp attached to the rear parcel shelf. The brace is not necessarily sexy looking but since its supposed to reside unseen in the dark corners of the trunk so I really don't mind. I'm definitely considering adding some sort of battery box/hold down to the brace so I can relocate the battery to the truck with a minimum of hole drilling. In the long term, I might duplicate the brace in beefier aluminum or stainless steel.
Rear brace on my desk at work
An end plate
A view of the end plate frrom the side
Yet another angle