I have mounted SP12F behind M1KPNAV.
Disclaimer: this may not be acceptable where you live.
I used Carlon 1-gang combo 1/2 inch|5/8 inch plastic mud ring.
If your local home store does not carry this particular type, you can glue two 1/2 inch mud rings together using pvc primer/cement
Trim the width to comfortably fit through the inside of another mud ring along the red lines in the picture in this post, but keep a lip that will rest on the drywall from behind
Cut a 1-gang hole horizontally in the wall accurately to fit the mud ring.
Drill a hole in the SP12F at the spot where there is a ring holding a microphone in some versions.
Get the M1KPNAV pigtails through the hole in SP12F from the front.
Connect your wires including the speaker.
Then the difficult part is to get the mud ring and speaker inside the wall. While half way in, using two short screws secure the SP12F to the 5/8 inch side of the mud ring facing front. (If you glued two half inch together, either side will work)
When the speaker is secured to the mud ring, sort of insert the mud ring in the drywall hole from behind and with screws attach the M1KPNAV bracket
As the bracket is bigger than the 1-gang opening, it will hold it in place.
The advantage of the mud ring is that it has screw holes on both sides, by design on the combo version, or when you glue two together.
This way you can simply secure the speaker to one side and while pulling it forward secure the M1KPNAV bracket from outside.
If you have a helper, it is easier. If you do it alone, which I also did, it helps to secure everything with strings so if they slip out of your hand they do not go down in the wall.
The mud ring with speaker attached on the outside does not fit through the drywall opening, hence the need to screw it in place while halfway inside.
You could try to use only one 1/2 inch mud ring and two 1/4 screws securing the speaker two one side and the bracket on the other side.
Note the SP12F is a bit wider than the 1-gang box you create from the mud ring, so if you place the hole in the drywall right next to a stud, the speaker will not fit
It has been a while did mine this way and they sound OK. my speakers are on M1TWA and had to reduce the loudness.