Make a wooden "plaque" large enough to span the T-bracket, screw it to the post, then mount your T-bracket/QS on top of that. You can paint or stain the plaque to match either the post or the speaker itself, whichever you prefer. The "ears" on the T-bracket are a too flimsy without a couple of screws on the outside to supprt them otherwise.

Half of communication is listening. You can't listen with your mouth.