I did this last year on my solar project. Instead of a junction box, I just used 2 PCV strain relief fittings and sealed around them with silicone. The #2 cable fit perfectly and its fully watertight.
Inside the basement, I cut a hole on the front which gave me access to the back of the diamond plate. I then put in a low voltage boxe and modified a blank plate with holes and grommets to protect the cables.
It worked great and looks good too.