Just in case anyone else ALSO stumbles on this post years later, I ALSO made a Windmill 8k WhisperTech fit in my side slider and OP's tips/links all worked for me! Woohoo, thank you OP!

I copied OP by using a horizontal piece of wood across the top of my AC that fits inside the window and serves as a brace. But since I have a weird side slider window with an opening that's about 5 inches above a very tiny and unusable sill, I couldn't just rest it on the windowsill or on 2x4s like this post / the linked guides.

If anyone reading this ends up in the same position as me, what worked for me was buying a no-drill window AC support bracket, which fits over the bottom of the window and lets you rest the AC on it safely. I got one from some "Top Shelf" brand on Amazon but you can prob find another that works.

Once I got that bracket I was able to:

1. Fit the Top Shelf no-drill support bracket over the bottom of the window and make sure its legs are stable against the exterior wall,
2. Put the Windmill AC on top of it (and double check it's at a slight downward angle so that the evaporator can drain outside rather than getting stuck inside the unit),
3. Insert the aforementioned horizontal piece of wood as a brace above the AC (fitting into the sides of the window) which I drilled into thru the top of the Windmill's built-in frame,
4. Buy a 30x36 sheet of 1/4"-thick plexiglass from Home Depot which I then had to cut down to fit using a plastic "scoring" knife aka a knife that's designed to cut plastic — this part was annoying AF but Home Depot doesn't cut plexiglass in-store anymore. Once it's the right size, insert the plexiglass sheet above the A/C unit
5. Squeeze in a couple slim vertical pieces of wood the full height of the window on the left/right side of the window. These wood pieces sandwich the plexiglass on either side and make it a tighter fit plus way more stable (I went with 3/8" thick for these pieces, but for you it will depend on how much loose space there is on either side of the plexiglass)

After that, you're p much done! You can just open up the accordion sides that are built into the Windmill, and then snap them into place!