How far out in the middle is the middle? Get out that concrete saw and drop a conduit down into the slab. Run it up a side wall someplace where you can feed it with power. As far as the horn goes, Bolt a 2-1/2" floor flange to the floor. Screw in a 10" length of 2-1/2" grc. Run your wiring for the horn up the middle of that in MC . Think different.
