Here what I cooked up along those same lines. Haven't had a chance to try firing it into the frame yet. Hopefully it fits without having to cut my 380mm seat post on my 54cm frame. I might also add some electrical tape for added security.
I didn't tie mine to the seat post, I used the ribbon only to pull it out of the seat tube. The ribbon makes it so you can pull the battery out with no hassle. Without the ribbon, I had to use a wire clothes hanger to push it out from the bottom bracket!
Also just to add more details about my procedure. I'd avoid any tape on the ER9/X battery holder because the seat tube is so tight. The only thing I added was rubber cement (with time to dry) after placing the battery cables into the battery holders for extra security. Once dried it adds a bit of bonding and it's easy to remove. I'm not fond of electrical tape, because after a prolonged period of time the adhesive gets gooey.
I had to cut my seat tube because it was too high with the battery installed. Without the battery I was able to adjust the height to size. If you have to cut the seat tube, I would cut conservatively until you get it down to the right height. I would also use fine grit sand paper after cutting to smooth out the edges.
