OK you'll need
2 x anderson plugs - the 50A variety should be more than sufficient they are worth $12.95 ea from Jaycar Anderson Plugs
5.5 - 7 meters of 8ga cable which is $2.50 per meter 8GA cable
& Eyelets to suit 8GA
you'll need to work out the number of eyelets required as it will depend on what fusing or circuit breaker system you go with and if you decided to use a proper dual battery isolator or not...
Battery Box well that will depend on the battery size you go with but around the $25-$20 mark should get you a battery box to suit.
Battery - well go talk to your local battery specialist or car audio shop. I recomend the Exide orbitals or Optima yellow tops the exides are cheaper and IMO a better battery than the optimas are now... You can also keep an eye out on ebay for a high AH deep cycle battery - they come up from time to time secondhand from UPS systems.
You have to decide if you want to use a isolator or not. if its something your only using occasionally then I think you could go with out the isolator but remember to manually isolate the rear battery when you stop or you'll flatten your main battery and then you'll be stuck
There are many types of isolators out there. I suggest the Redarc isolator. But scosche make a similar isolator which is a "dumb" variety in that it makes contact as soon as the ignition is turned to on. theres also some cheaper ones from jaycar and other places as well. you wont get much change out of $140 for a redarc isolator though!
I also suggest that you use circuit breakers
- manually resettable ones, rather than fuses. this way you can manually isolate the rear battery plus you dont have to "replace" fuses in case it does trip. remember to fuse within 45CM of BOTH batteries.
I dont see you getting out of it under $150 (plus battery and optional isolator) for the cabling, battery box, terminals and circuit breakers required.
Ok now for the method...
to be continued