These mounting plates are basically the same shape as the fixture brackets, but 1/2" bigger all the way around. The jigsaw was used to cut a large hole out of the center so the plate would fit over the fixture box. Then the plate got screwed to chiseled-out spots in the logs so the plate would be plumb.

The front of the plate is about flush with the front of the fixture box, so the light fixture bracket could still attach to the box and be tight against the plate. After both fixtures were tightened down, Jeff siliconed around the bracket and plate to seal out water.