Thanks to Linda Frost for a swell idea! Sitting on my back porch, Linda asked if the privacy fence around the back garden belonged to us or the neighbor. When I told her it is ours, she brightened and said she had seen a photo online of just such a fence with marbles in it. Not long after she left that afternoon, Linda sent the link, and I was pretty knocked out.
Yesterday my husband helped me drill holes in a section of our fence where afternoon sun streams through the slats. We made two basic sizes of holes and then customized most of them with a burr to accommodate the somewhat-less-than-uniform small and medium marbles. Here is the happy result: