What's the best way to start designing a NEMA grade enclosure?
If someone has previous experience in designing that I need help to understand the overall layout and design of the enclosure.
Hoffman and Saginaw have NEMA Enclosures in 3D files available (step I believe). You can start with one of theirs and modify as necessary.
Becoming familiar with NEMA specifications for the enclosure type you are going to design is the best starting place.