PLANTED do a z4m specific seat bracket that comes with the seat belt and seat buckle mounts pre welded, as much as i love the VAC and similar options but the price point is insane for those pieces imo, plus you need all sorts of accessories to get the stock seat belt to work plus the reciprocal. your talking a $1000 easily.
i hope these brackets workout as they are really nice pieces and the price point is fantastic, ive got a set to fit my carbon fibre tillet seats , using tillet slider and a slight taller mount.
my mounts are sat in my workshop ready to install when the weather improves here in the UK and they seem solid quality.
https://www.plantedtechnology.com/pl...ger-right.html