I have a 2x6 car decking ceiling with a 2x4 cavity above through which to run electrical and plumbing. I'm puzzled by the best type of box to use to mount light fixtures in the room below. Obviously I'm going to want to cut into the ceiling to get the box flush with the bottom surface of the T&G, but I will then need to fasten the box somewhere in the ceiling. Most boxes only want to account for 1/2 wallboard, and I effectively have 1.5" of material to deal with.

Is my best bet going to be a ceiling fan type box, with a 2x4 between the cavity joists?