I used a Marine battery box. Cdn Tire sells one for $15 (link below).
http://reviews.canadiantire.ca/9045/0790312P/reviews.htm
The fit of the battery in mine was a bit loose so I used dense foam as padding - pieces of “pool noodle” actually becuse its made from chemical-resistant polyethelene foam and becuase they cost $2. I riveted the box to the trunk floor and it is solid. I might not do this again though as now I can’t remove my main trunk carpet to wash it… I also recommend thinking through your wire routing. Protect it against sharp edges or wear points to ensure you won’t have reliability issues (or start a fire).