I 've seen quite a few cars lately with aluminum fuel cell, and sealed battery box in the hatch area without a rear firewall.
I always thought the rules stated that a firewall was required with battery relocation and/or fuel cell within the driver's compartment. I know the rule goes way back to before aluminum fuel cells became commercially available...

...did the rule change?

 

I believe there are a couple battery boxes at are legal without a firewall. A fuel cell still needs to be isolated though, but it can be an aluminum box around the fuel cell with a flip door on top to access the fuel filler cap, it doesn't have to be a full firewall.

 

pic removed

 

I posted a response to your thread on the Bullet, but to recap here, that picture was taken right before we fabbed up the cover for the fuel cell.

Chris