I've seen aluminum bulkheads for fuel tanks in the rear,
as long as it's sealed completely I don't see why it would
be a problem. The easiest way to accomplish a bulkhead
is to mount the cell in the stock tank location and build
an access flap for filling up, or mounting a small cell in
the engine compartment. We use a 3 gallon aluminum
cell and normally get 2 runs on that.