As usual, inspiration hit me as I was perusing the isles of the local hardware store taking mental note of what I see for use in future projects and keeping an eye out for stuff to use for those fledgling projects-to-be always floating around in the back of my mind. I saw these pebble mats meant to be used in the sink and realized they were what I had been looking for.
The mat I got was something like 14" x 9" so it was enough to cover the sill, but would require some editing.
The first step was to grab some paper scissors and tape to make a template of the space. I wanted it to look like one piece, so I tried arranging the mat over the template several different ways in order to find some areas that both fit the space and fit together without requiring too much cutting through the pebbles.
I found some good chunks and marked them on the stencil and then taped them off on the mat and cut them out. The plan was to glue them together but this didn't work out so well because of the minimal surface area connecting them, but it ended up not really mattering since the pieces fit fairly snugly and the stuff is kind of rubbery so it doesn't move once you set it down.
If you have read this far, congratulations for having an exceptionally long attention span, because the title and pictures make the idea pretty self explanatory. Now get up and go do this yourself if you have a good spot for it, and enjoy.