What would you want to see for a professional, water tight installation of a light sconce on wood board and batten siding like this? Should there be a mounting board that has caulk sealing the gaps in between the siding boards? If the fixture was mounted to the siding directly, would caulk around the fixture’s mounting plate be sufficient?
Thank you.
A weather-tight box is the way to go with caulking on the top and sides none on the bottom so if water gets in it has a way out. 
Thank you for your reply. Can you expand on what you mean by a “box”. Are you referring to a piece that would be on the light fixture or something that would be attached to the siding/sheathing of the home?
With this particular siding, the electrical box can be mounted directly to the siding.
Oh, you’re talking about the j box. There already is a box installed in the wall to house the wiring… My question is about a light fixture being attached here and how it should be sealed.
See this picture:
The top and sides should be sealed.
I would add a filler piece above and below in between the boards, caulked properly, to have a smooth surface, pull that box out and replace it with a water tight box as Scott suggested. 
Then the fixture would be on a flat surface.
