Be interesting to see how the battery holder turns out. Really good idea for a bike with an extended swingarm. Looks like you have the skills to make a steel swingarm a few inches longer. Then you could carry along the seat tube with no problems. Maybe a project for later? Allows the really crazy strong pinch dropouts too.
Make it support itself as much as possible on the frame. I've bent more than one seat tube, and broken one.
THE LIPO RULES. NEVER ABOVE 4.3V NEVER BELOW 2.7V DON'T PUNCTURE
Ideal charging /discharging range for Lipo, 3.65v minimum 4.1v maximum
See battery technology section, FAQ thread at the top of the page for lipo noob info.