At WEVO we have done a 911 Battery box in molded fiberglass that goes into the aircon cavity.
We searched high and low for a suitable battery, as the molding and clamp are specific to the shape of the battery.
We elected to go with this one, made in CA, gel, very inexpensive and plenty of power for any of the applications most Porsche users would have.
http://www.teammiata.com/TM420.htmThe battery weighs 23.5 lb and we have a billet clamp machined for the lateral groove in the top of the battery that could be used in other applications if that is of interest to anyone.
regards
Hayden
PTBT