I bought bus bar from
Discount steel. Holes are drilled and tapped in the bar. The copper bar is attached to pieces of HDPE I had around and those pieces are epoxied in place. I used standard weatherproof 4" utility boxes.
You need two bus bars if you are organizing both the positive and negative phases of the 12 V DC system. If your negative phase is split off to the frame and organized well, you only need one bus bar. Bus bars are also sold premade or a guy can buy a lug terminal strip to handle the connections.
Good luck with your project!