When I do that sort of modeling, I don’t bother modeling those parts flat as they are shown in the plan you’re working from. I just model them in 3D from the start. It’s not that difficult to do with all of the dimensions that are provided and it’s a whole lot less work.
Joint Push/Pull can be useful for making the thickness of the sheet metal.Also consider starting from a cross section or in the case of that part, start with the closed end as a rectangle. Extrude it to 0.5 thick, use Offset to outline the top and long sides of the box and extrude that.
Also consider the order in which you add the small details like holes. I wait to add holes that go through multiple parts until they are in place relative to each other. It’s then a simple matter of make “drill” or “punch” components to cut the holes quickly and precisely using Bool Tools 2 or Enroth Solid Tools. I did that for most of the holes in this traction engine model which I need to finish up.
