I completed this mod with some great success. I used an AGM battery, and it works well. It has enough CCA to turn the bike over easily, but significantly less Ah. Don't leave the battery on for more than 5 minutes and you'll be fine.
The Li ion batteries that are now available should be even better choices. You should be able to find a battery with comparable CCA and Ah to the stock battery in a similar sized package to the AGM batteries. Some of them have special balancing circuits already installed, so they don't require any special care. The only thing about the Li ion batteries is don't ever let it FULLY drain. One time will kill it.
Back to the mod... I kept it mine on the swingarm bracket. It's made out of 1 1/4" strap steel. Rather than welding threaded rod to the bottom, I pressed a carriage bolt through the steel and ground most of the head off. I didn't weld a strap to the back of my battery box, so when I have to take it out all I do is remove a zip tie. Use some rubber like a bicycle inner tube to insulate the battery from vibration. I also did this mod WITHOUT extending any of my battery leads. The negative lead was relocated to one of the former battery box mounting holes. For the positive lead, I relocated the solenoid onto a bracket I made from the left side of my air box.
A clear shot of the solenoid:
The wiring all tucked away neat. You can also see where my negative terminal is frame mounted:
A look at the outside of the bracket. Here you can see where the hardware mounts:
Any other specific questions?