I've been thinking of doing that same thing. A parallel brace to hold up the magnet and a perpendicular, vertical brace to couple the driver to the back wall. (The SDX-15 is pretty deep and heavy, so it puts a lot of leverage against the front panel without support) Tolorances need to be very tight and my tools aren't the best, so it'll require some skill and likely a lot of hand sanding to shave 'em down
I was also thinking of using gasketing tape between the motor and any wood, just in case.