What I'm looking at doing is dropping in a very small metal battery box next to the frame rail on the passenger side floor.. with a Zeus fastened cover.. my lithium battery is only about the size of a quart of oil. technically the battery would not be in the passenger compartment.. but it would be accessed through the passenger compartment
When putting this sort of thing in the trunk there is a spec for bulkhead material. When I put the fuel cell in place of the spare tire in my '68
barracuda I wanted to keep the trap door functional so I just made a cover for the cell that attaches to the floor. There's an access door for filling right in the top. Never had a complaint from a tech inspector. When it came time to put the battery in the trunk I just made a cover that's held to the floor with wing nuts. The battery hangs through the floor about half-way so venting isn't an issue. If you cover it in the proper bulkhead material and have the bottom open it should pass tech but I won't guarantee it. Tech inspectors are a crapshoot and usually not technically literate.
