For my example I’m going to use an 8- x 8- x 8-inch sheet metal fitting called a ceiling register box, or C box, just to layout the dimension you’ll need. The first two dimensions are always the length of the sides of the duct fitting, and the round collar is always the third dimension.
As you can see from Figure 1, you want to order the C box according to the size of your roughed-in hole. Your C box will likely have either nail fins, or a plaster flange that goes all the way around (see Figure 1). You can also order the duct fitting “raw” which means it’s just a raw edge without any flanges or stops.
The plaster flange is going to add about one inch to each side (Figure 2). This flange is supposed to go above the sheet rock. If you’re installing the C box snug against a rafter, it’s best to nail the box directly to the framing from the inside. Also, don’t forget to spray paint the inside of your box black before installing. This makes it so you can’t look up through the register and see shiny sheet metal.
The diffuser is also sized off the dimension of the hole. In other words, an 8- x 8-inch diffuser will fit an 8- x 8- x 8-inch C box. The visible side of the diffuser is purposely oversized to hide the hole more completely. Figures 3 and 4 show an 8- x 8-inch diffuser. Figure 5 shows a complete 8- x 8- x 8-inch C box with 8- x 8-inch diffuser.