Window A/C unit mounting

Just wondering I have a new goldstar window unit that I am trying to figure out the best way to mount. It's going in a horizontal sliding casement window with an opening of 27" with an .5" lip to hold the window.

The unit itself is 14.5"x18.5"x12" So far I had just thought of making a platform that sits over the window case and screwing it down to the sill, however it's very back heavy. I am planning to use 5 decent size screws to mount it to the 7" sill but would it be in my best intrest to also make some type of feet to sit on the outside of my wall? Please if anyone has any better ideas I'm all ear's.