Ok, this is what the octagon stock is for:

http://www.beamfoundation.org/media/KBow_NIME.pdf

2nd page shows the electronics and housing. Just above the
K-Bow legend is the frog adapter that I make. It's hollowed
out as much as we can get away with and fits over the
octagon shaped carbon fiber bow.

The octagon part I'm looking to make is at the end of the
adjusting screw. I was given the option of knurling this,
but even with a CNC grade knurling setup, I find getting
good knurls to still be something of a dark art. I can
produce knurls acceptable for most needs, but we're looking
at a high dollar item here, in market where craftsmanship is
still highly valued. Sticking with the octagon shape
maintains some tradition. I will be machining multiple
grooves mainly for weight reduction, though it will put a
high tech edge on the traditional shape.

Back up on the URL to the base site and there is an embedded
video link of it in action. I don't have quicktime installed
so haven't been able to view this myself yet.

Jon