I used a sheet of .063 Al screwed to the bulkhead behind the baggage area (just behind the corrugated baggage bulkhead) and then put the battery box and master relay on the back side of that sheet. This ultimately requires that you run a large cable from the battery to forward of the FW, plus a wire for the master switch to the relay. This takes about 15 pounds off the firewall and puts it considerably aft of the CG so will help a nose-heavy condition. It does add a couple pounds extra weight because of the 10-foot heavy copper cable.
There are a couple threads out there that describe similar solutions.
PM me if you want more detailed instructions, but I'm traveling right now and won't likely get back to you until after the weekend.
greg
