I can understand your concern, and have had the same thought. (there are two areas in my home I'd like to install the in-wall speakers)

I'd build a box and install them in / on it. Build it out of 2X4's, 16" wide, 24" high to mimic the stud space where you will need to mount them. Use some silicon and screw some thin plywood or other sheething over the studs. Then just hang them or support them at the approxitmate height off the floor where you plan to mount them.