First, get a switching power supply, probably around 20 amps (yes, a bit much--but for the requirement of the aux. 12v tap) and instead of using a high-amp switch, use Anderson power pole connectors. Possibly the Samlex SEC1223 would suit the purpose--or its equivalent. The reason I specify a switching supply is the weight of a regular power supply--if you needed to cart it around.
Just my preference, but I'd set it up so that the power supply would be outside--so you wouldn't have to have it there if you didn't need it, that's where the powerpole connectors come in. One connector on the pigtail out of your box, one connector on the power supply output leads and one connector on your battery.
You could also set it up so that you could use a connector cord right out of your vehicle with powerpole connectors on both ends of it. That cord should use ten gauge wire, the others could use no smaller than twelve gauge--ten would be better.
First, thank you very much for everyones input!!
Wow i was thinking even higher for the powersupply...like a 50A...haha and actually a major detail that was left out is the first stop of the mail power will be to a water tight fuse panel. The reason for the powersupply being internal is that in an ideal situation i would also take the box camping with me and we would run it off a generator. And im just a neat freak and want it to be internal haha.
The plan was to use power pole connectors however just have a socket on the back of the box and have a pig tail made for my car.