Let me get this straight,
You have full access to the speaker behind the wall? If so, that would require a speaker specifically designed to be infinite baffle (IB) because it is not an enclosed space. Most inwall speakers have the rear wave coming out of the back of the speaker contained by the wall--no problems with the sound creeping around and creating phase issues with the front in phase portion of the sound. Now if you use the backer boxes to prevent that sounding blasting out of the back--and you have access to the speakers--realistically, you can use any typical box speaker.
OK, this is how I would do it. Say, just for fun I liked a speaker that was 18 inches tall, 9 inches wide and 10 inches deep. I'd cut out something like a 22" X 12" hole in the wall then mount a grill over it. You can purchase speaker grills seperately or build them with a piece of 1/2" plywood and stretch whaver acoustic cloth over it. Mount it with glue, a few finishing nails or magnets if you want to remove them on occasion. Go behind the wall and place a shelf at the bottom of the cut out and put whatever box speaker you like on that shelf firing through the grill. This will allow you to angle the box up/down/left or right to get the correct setup that works for your room. Once that is done, simply take denim insulation, pillow stuffins or whatever and place in around the edges of the speaker so it seals off the wall cutout to the speaker box. Get it flush with the wall then put the grill back on.
What happens when you do that with a regular box speaker? Well, it changes the bass response because the speaker baffle is not small allowing the bass to "wrap around" and go omni-directional at a certain point (called baffle step) so you'll see a rise in bass output below baffle steup or around 250 to 400Hz on down. Most stand alone speakers have the crossovers and tuning done to allow them to stand alone so use "baffle step correction". There are ways to remove that from the crossover but--now that you have room correction, just let it do it's job and trim down the increased bass when applicable.
Think of it as an "inverted bookshelf" that blows through the wall, not on the wall.
Don't forget to put the insulation around the wall cut out to the speaker sides. If you like, it is rather easy to do by using old denim blue jeans and stuffing from an old pillow. Make long pillows that are wider/taller than the cut out and slide them in to seal it off. The pillow stuffins are inside the denim so a way to prevent them from looking weird/odd when viewed from the back. If you can't sew, you can make them with a hot glue gun, denim and pillow stuffins--just hide the seam. Hot glue gun makes wrapping the acoustic cloth around the 1/2" plywood frame quick so "acoustic pillows" can be quickly made at the same time.
I'm kinda a weird guy though, but if it was me that is exactly how I'd do it if I had access to the other side of the wall. There are companies that will print out pictures on acoustic cloth so your speaker grills can be movie posters, pictures of your dog or different patterns. Very common for basement theaters to use those printed acoustic cloths to make noise absorbers with Owens Corning panels which are framed with 1 X 4's and covered with the acoustic cloth "posters" to stealth them out.
Just another option, as long as you have room correction the increased bass response below baffle step can easily be sorted out and will be more efficient--nice bonus. In the future, it allows upgrades by simply swapping speakers instead of starting over. If you don't have or desire to use a jig saw and glue, there are companies that make acoustic grills in whatever size you desire--you can get frames custom made if you like. Don't let the fear of messing up making a grill remove that option--if you can cut a hole in the wall, you might as well cut some 3/8th ply, grill cloth and glue gun your own grill to match.
Good luck with whatever you decide and enjoy your new upgraded system.