So what's the problem? I do it the "old fashioned" way...use a 4-square box with a (round) mud ring appropriate to the drywall. I cut the hole, go to the attic, set the 2x4 in place over the box, then return below to anchor the box. With luck :-) tha't my only attic trip.
I use the Westinghouse version of the twist in place brace and it comes with a 2 in deep box. After I set the brace I give a quick hang on the bar and have never had one move.
