For the hardware there are many options.

My gut feeling is to go for Aluminium extrusion; easy to assemble, and  it'll be easy to mount components to it.

Assembly will be rather trombone like, built from two, elongated U-shaped parts.

The first part will be the slide mechanism. Rather than an assembly that moves, there will be a thin bar that will slide inside the extrusion.

Moving this will adjust the a potentiometer of sorts (still undecided on this element).

The second part (not as long as the first shape will join the 1st part at a 90 degree angle, giving that trombone shape.

On the other end of the U shape a bracket will be created to hold a speaker, for on-board output.

A DIN rail will be mounted through centre to allow for an easy and modular electronics section.

The electronics will comprise of:

Battery Pack outputting 8v-24v

Pi/Tinkerboard with Justboom Amp and DAC  combo.

Arduino 

The Justboom Amp accepts 8v-24v via it's barrel connector and powers the amp, DAC and Pi from this input.

The speaker connects to the amp hat also (either 4Ohm or 8Ohm. I'm going to be using a car speaker at first).

The Arduino will read any buttons and potentiometer values and use those to send MIDI information via USB to the Pi.

The Pi will be running software to interpret that midi data and turn it into sound using a VST or other virtual instrument software.

Depending on the software I use I may either add an Arduino lcd/button shield to control sounds and settings, or I may use a webserver and change settings via Wi-Fi.