If you will permit me to add my 2 pennies regarding small-of-back (SOB) carry: As an Instructor, I strongly discourage any of my shooters from carrying a pistol this way. It can easily cause damage to your back and spine. Not just if you happen to fall on your back, but even from sitting for long periods of time. I'm sure there are advocates of SOB carry on the board, and I don't mean to discount their training and experience, but I recommend against it.
I would think that a Mustang could easily be carried in a good ankle rig. Don't go cheap. Buy something well made, stable, and with good retention. I lost a Taurus Model 85 many years ago during a foot pursuit up the side of the highway. I was using a cheap, elastic band type ankle rig and never realized it came out. A citizen pulled up where we had caught the guy and brought it over to me between his thumb and forefinger, like he was holding a dead fish or something.
Embarrased? You betcha'.
Gonzo