I've just put an order in for the following lengths of 20x20 aluminium extrusion:

500mm x 2

100mm x 4

This will give me the parts I need to:

• Build my slide
• Mount my Arduino Pro Micro
• Mount my Air Pressure Sensor
• Mount my encoder for pitch changes

Once the slide is built I should have a fully working slide-based MIDI controller :-)

Following that it will be the construction of the back frame.

I am contemplating changing the frame from a simple 2D section to a 3D box section, allowing me to mount a forward facing speaker for a more trombone-like output. 

I could always add a secondary speaker at the back of the frame behind the main speaker for some interesting chorus-like effects or stereo output (playing harmonies, stereo delay etc).