Meny people just build a shot wall and attach that to the ceiling and run a 2x4 along the wall. A few blocks between the wall and the the 2x4 to keep it straight. We found that often the ceiling joists are up and down and this is transferred to the box below so now we attach a 2x4 to the ceiling and come down with a peice of plywood or osb and attach a lower 2x4 to that on the inside and you can put that in level and level across to put a 2x4 on the wall and then the few blocks to keep it straight. For a double we just start with a 12" drop and than do it again with a 6" drop.
Keep in mind, if this area has not been drywalled yet, you do want to stop loose insulation from entering this cavity with poly or something and the wall should be firestopped. Any fire in the wall wants to be stopped from entering this drop. You could drywall first, or solid block between the studs in the wall or put plywood between the wall and your box.
I hope this helps
