I probably should have kept a buildlog of this while I was constructing it. Oh well.

Here's a video of the final product doing its thing: https://www.instagram.com/p/BU9EzlRAwhg

This was a reasonably simple build, and the trickiest part was getting a sufficiently stable 5V and 12V supply across the range of voltages output by my batteries. I originally attempted to use a SLA gel-cel 12V (13.xV) deep-cycle battery, but toward the end of its discharge the voltage would droop below what the UBECs were able to work with. The higher starting voltage (16.8) and rated voltage (14.2) of 4x18650 gives me stable performance throughout the batteries' life.

The components are assembled and bolted down with plastic standoffs inside a laser-cut acrylic enclosure built with one of the many online laser box maker sites, and the pushbuttons are attached to the top of the enclosure, directly under the backpack's "controls." All work and provide pretty good tactile feedback. The buttons are wired into the GPIO header on the Pi0. The LCD is also protected with a "sandwich" of the same acrylic material as the enclosure is constructed. There is a dedicated amp for each speaker channel, and the speakers are positioned on the bottom of the sides, pointing outward, to provide stereo audio. The sound quality is good, and can be quite loud when turned up to max. The backpack itself is unmodified apart from a ~2cm hole in the pocket behind the screen to permit routing the LCD cable up from the box. The batteries rest on top of the box, and the backpack is still about 60% usable for cargo.