Rabbit, this is a ongoing issue in many areas.
The UL white book may cover some of this.
I worked in a industrial situation a few years ago, and this manufacturer made their own stainless steel cabinets for controls etc...
and were UL approved in house.
We contract a local metal fabricator to make "highhats" for installations where 90's can not be turned down into gear.
The listing situation is not a requirement, the AHJ can and must inspect items that are not listed, but that doesn't mean there is anything wrong.