I have a Wiechers bolt in roll bar for the z4m. Brand new and unused. This was custom ordered through Wiechers with the x-cross bracing and the shoulder harness attachment point designated for the rear horizontal cross bar.
This is likely the last company making these bolt in bars for the z4m chassis. See link for install:
https://www.zpost.com/forums/showthread.php?t=1489265
$600 + shipping OBO, located in Seattle, WA.