Thanks for all your ideas. Although I didn't use any of them per se, they did help me think through all the factors. Here's what I did. I surface-mounted the fixtures on a 6x9 paver block using Tapcons. I leveled the proper spots in the rubble below the concrete walkway and placed the blocks. Then I piled rubble around them. They seem pretty stable and yet can easily be moved or reset. (I have to get some bronze touchup paint for the blue Tapcons.) I'll have to get some night photos to post here. The dock association is very pleased and want me to continue spots farther around the quarry wall.