As I look for alternatives in the speaker department I've toyed with the idea of mounting an old bluetooth speaker (with Aux in) to the end of the instrument, just so I can get building and playing. The battery would need replacing, but given it's powered from a 3.7v old Nokia-style battery it may well be powerable via a Pi USB port.
Using this style of speaker would greatly reduce the need for a wide, boxy frame on my shoulder and would allow me to really condense the design.
While thinking about making the back section as compact as possible it got me thinking about the slide.
The plan was originally to build a frame roughly to the size of a normal trombone, and the player would place his hand within the slide frame to manipulate the pitch of the instrument via the ultrasonic sensor in there. Now I'm thinking... Why build the frame?
The frame would have no practical impact on the instrument, and would make the thing 2-3 times longer than it needs to be. In fact removing the frame would give the player more rooms for gesturing. I could add another ping sensor to pick up the hand moving into and out of the line of sight of the main ping sensor. This could be interpreted as either a slide up to the note or away from the note, or could be used to change ranges.
This has potential.