I've been looking at few options but all of them require at least some removal of rear seats.
AE makes one, which bolts to the rear seat belt and rear seat deck. (white)
Evolve from UK makes one, which bolts to the rear bracket holes. (red)
The harness bar on M3 GTS seems to bolt to the rear folding seat mounts and the rear seat deck. (orange)
None are easy to remove. Perhaps try the buckets with regular seat belts first? I've been doing some karting lately and they don't have any seat belts, but the bucket seat alone held me in pretty well.
2011 E92 M3 DCT
1991 Spec Miata
2008 E92 M3 DCT - KW Competition 2-way / Brembo 365 BBK / OMP ARS racing seats / rear seats delete / GT4 aero / AiM EVO4 data logger / SmartyCam racing camera
2008 E92 M3 6MT
2008 E92 335xi