I really hate this cable system. When the cable gets muddy, you either have to clean it carefully prior to stowing it or end up pushing mud into the trailer. Worse, when the weather is very cold, the cable gets stiff and is impossible to shove in the hole. I also find it tedious when I need to quickly connect shore power to the trailer at home.
One of the challenges was the size of the hole shown here. It is too big for either of the two conversion kits available so I had to fabricate some sort of a plate.
I made the large hole by drilling a series of small holes along the outline, bashing out the bits between the drill holes with hammer and chisel, and filing the inside smooth.