So, I assembled a little frame to start mounting components to using the Totem Maker parts I have.
Unfortunately the car speaker I have is really weighty and is just too heavy to by hanging in front of a player's shoulder.
A trombone's weight is pretty evenly balanced either side of the shoulder, with weights available to help counterbalance the instrument if needed. I thought about using the mounting of the electronics and batteries to counter balance the speaker but it still wouldn't match the weight of it.
I'm now on the look out for a smaller speaker.