Why do you try to model this object out of one box? Take a closer look at your reference. You will notice that this beamer is made out of different parts. It is easier to model those parts as seperate objects. Second point, take a look at your "lense hole" its really wonky. I guess it would be easier to start over again…
You can do this a few ways. 1) Object Painter Scripts like Advance Painter or Soulburns Object Painter Script. (I personally like Niels script a lot better, more stable, more features, more accurate placement) 2) Attachment constraint. 3) Select an edge, click "create shape" this will give you a spline. You can then path…
In this case more geo is a better solution as the shape doesn't smooth very nicely but you can select edge loops and use creasing also on any subdivided mesh. *Object on left has no creasing and object on right has 0.5.
Yeah, loft works too. And it's automatic procedural, don't have to analysing model/object at first. But doesn't hurt to model it like every other low powly objects, just need fair edges plan and such. Thanks for loft method suggestion.