Irregular roof pitches, unusual attic framing and tight attic access can make cutting the angles to frame skylight shafts a nightmare.
Instead of wood, try framing the shaft with light-gauge (25g) steel.
Because the steel studs fit inside a U-shaped channel at the ends, called the track, they do not require the precise angles for joining that wood does. A single carpenter can stay in the attic to cut and frame the shaft with aviation snips, a cordless drill, sheet-metal screws and a couple of clamps.
If you have been considering steel framing, this is a good place to start — engineering is usually not required for framing nonstructural steel-stud assemblies, and the job is small enough to not worry about the time invested in the learning curve. And your skylight shafts will be straighter, easier to frame and have fewer drywall cracks from shrinkage.
Be sure to get both the stud and track when ordering. Steel studs commonly come in 10-foot lengths and in widths of 15/8 inches, 21/2 inches, 31/2 inches and 35/8 inches. Regular sheet-metal screws will work with 25g steel; however, self-tapping screws make the job a bit easier, especially when learning. The same goes for attaching the drywall.
Always use safety glasses and gloves when cutting and assembling steel products.