Summary

The Supermicro 836 3U chassis back panel conversion piece for full-size cards on risers (e.g. X9SRW) as seen on
https://www.wanhunglo.com/2020/fitting-a-proprietary-supermicro-x9srw-f-into-a-standard-3u-836-case-p23

If you can, please use the RSDOC (RS DesignSpark) file as that's my original source file. The STL is just an export that might not have had the perfect settings for your printer and/or application ticked or might even be incomplete.