Trunk bulkhead thickness?

I put a fuel cell in my trunk, and remember seeing in the rules that you need a wall between the trunk and passenger compartment if the cell is in the trunk. I can't find my book to look it up, but I was thinking it needed to be 1/8" thick, so I bought a sheet of 1/8" aluminum, but it looks like it's a lot thicker than necessary. Anyone know the requirement?