If you've got money, another option would be to go with a lithium battery vs a lead acid. (Google ballistic battery for one brand). They definitely are not cheap, but then they have a lot of pros:
- Smaller than a lead acid battery (meaning you can fit a larger capacity one within the same 'footprint' of a lead acid battery).
- Significant weight savings over a lead acid battery.
- Shouldn't lose its charge as quickly as a lead acid battery, assuming you have no parasitic load on the battery (alarm, other electronics, etc).
The one thing I also don't like about Binoy's solution (aside from the cutting), is the fact that it looks like the physically larger battery might also partially block/restrict the air intake, which wouldn't be good for performance (it might not matter though).