He probably doesn't need it to stick out 3.5". The waveguide + CD is the limiting factor. Including 1/2" drywall, he'll have a total of 4" (2x4 is actually 3.5"). The waveguide is 3.5" deep. The CD is 2.4" deep. I say 2" stick out is probably fine. I have 2x6 construction and mine fits fine. So 2" is all it should take. And that really should be ok. There will be some added diffraction but shouldn't screw things up to bad. Like I mentioned, he might want to make a 2" wedge for both sides of the baffle. The top and bottom shouldn't be an issue. All this assumes no rear panel like I did. And porting it will make things a little trickier. I went sealed.