On mine one box is positive and the other is the negative box. Unless you have a ton of stuff a bus bar with spots to hook four or five lugs is about all one needs.
I got my bus bar from an electrical tech friend, he gave me 2 pieces of copper and 2 pieces of aluminum about 4"x 8". I cut one of the bars in half and trimmed the length to fit in the electrical box. 1/4 X 20 holes were then drilled and taped into the copper, spaced appropriately for the fuses I was going to use on the bus.
I had some nylon washers about 1/2" tall from Lowes that I epoxied to the bus bar then to the bottom of the box. The washers may not hold up in the box but all they were needed for is to keep the bar in place while I hooked up the wires. The bar is cut tight to fit in the box so once the wire are hooked up the bar isn't going anyplace even if the epoxied washer fail.
I purchase boxes with threaded holes in them but if I was going to do this again I would get boxes with no holes and just drill my own so I could space the holes out along the box height so I didn't have as much trouble keeping battery lugs from touching each other. Follow westend's link to fuses as the ones I purchased from my local radio install place were longer and made things really tight in my positive box.
For connections to the bus bar I used 1/4" x 20 x 3/4" stainless bolt's, lock washers and flat washers screwed into the taped bus bar holes. The same bolts where used with nuts for fuse to lug attachments.
My next step is to put a little tape around the wire entry holes inside the box then fill each hole from the outside with silicone to seal of these entry points.