Mmmm $43.11/pair for the smallest 5.25" size. Perhaps still too wide for the ~4" stud cavity.

Having thought about it some more, I would just cut away some of the stud depth to allow a wider (than 4") speaker to fit, perhaps 2" of the 3.5" stud depth. Easy to do with an oscillating multi-tool, or perhaps a jigsaw or sawzall. Possible with a hammer and chisel too, but that might shake other stuff loose.