You might be able to find a source for the parts for a box like that by contacting the folks who build Roadtrek Class B Motorhomes. They usually have a box along the outside of the frame, behind the driver's door. It often contains the outdoor shower and controls, and the shore power cable on Roadtreks. I have a buddy who uses that outdoor shower to wash his motorcycle after muddy rides.
Here's a photo of the box on a Roadtrek, with the door open. I've often contemplated adding one to my van, because they are convenient and useful, and otherwise wasted space.