How to mount window air conditioner?

How to mount window air conditioner?
I got a used AC and want to mount it, but the window sill is 4 inches and the AC is 17 inches thick. It will just fall out the window and that accordian wont hold it in, I know that's just to seal the hole. Do they sell some sort of brackets, or do I need to weld up some giant L brackets and screw then to the side of the house? This is a trailer house, so the walls and sills are even thinner than a regular house, but it would still fall out of the window no matter what kind of house, and the window frames are aluminum so I cant screw to them.
Thanks for all advice.
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