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.
E92 M3 DCT