Reddit reviews Tinksky Stainless Steel 90 Degree Angle Bracket ,Corner Brace Joint Bracket Fastener, 20mm x 20mm x 16mm, Pack of 20

In order to keep the ac from sliding out or being removed from the outside, place a board under the unit on the inside window sill, 2-3 inches wider than the ac on each side, that will allow it to sit at a slightly downward angle going out, and screw the board down with 2 screws on either side of the unit into the window sill. Then use a small corner bracket, with one screw into the board, and another into the ac, on both sides of the ac, with a wood screw into wood, and a metal screw into the base of the ac, much like the ones pictured. You can place the board under the ac in the proper position, with the corner brackets, and mark it with a magic marker, so that the board and bracket can be attached to the ac while on the floor, and then placed back on top of the sill, if drilling into the side of the ac-bracket is made impossible by its windowed location. If the ac is screwed into place and made very secure by this method, the outside shelf may become unnecessary. You be the judge. You can also use 2 brackets per side. I sure as hell hope that makes sense. I'm exhausted. Good luck.

edit; drilling a small hole in the metal side at the base of the ac for the machine screw and bracket will be necessary. You’ll need no more than a 1/2” screw.