Frame inserts like grills are attached with M3 screws to the back side of the frames using any of an array of thirteen available M3 threaded screw bosses. Arbitrary additional functional layers such as decorative fabric, light show, sound system can be readily stacked up and attached by simply using progressively longer M3 screws to hold them in place. The body frame attachment mechanism is unaffected by these additions. For those who want to feel a “traditional” solid back on the guitar they are playing, plastic or carbon fiber back shells can be added as the outermost layer of the insert stacks. These are our first 3D-printed back shell prototypes, which revealed an error in the surface curvature design of the treble side shell. When making the CAD model, the surface curvature is created by overlaying a grid of control points over a flat surface, then pulling up control points to create a desired surface profile. Here the bass side was designed first, which came out very nicely, which was then mirrored to start the treble side surface. The treble-side cutaway created an asymmetry that had to be fixed manually, but wasn’t fixed quite right.  The subtle dip error in the treble-side surface curvature is visible in the surface reflection.