I also wanted to fill in the space between the grass with a low laying plant. I cam across some Mouse Ear Coreopsis 'Nana' on sale and so picked up three plants. These plants like sun to part shade but will only grow about 6-8" in height, just below the Moon Window opening.
The bright yellow-orange blooms look nice but will grow no higher than the opening.
Together with the grass, these plants will balance off the entire fence panel.