The hot season has lasted much longer than usual this summer, so we decided to install the old air conditioner we have lugged around unused for the past six or seven years. The challenge was that our windows open side to side instead of up and down. To make it work, I built a figure-eight shaped frame that slides into the window sill; the bottom half of the frame houses the AC unit while the top half has plexiglass.
The plexiglass is 1/8 inch thick, which just so happens to be the width of the groove cut by my table saw. Worked out well.
I covered the screws on the side with wooden plugs that I cut with a special drill bit. They always cut a bit too small for the hole, but I dropped them in water for a minute or so and now they are swollen to fit the hole: