I have horizontal-sliding windows, and lacked the equipment to mount the air conditioner on the edge of the window (besides, I didn't want to be the only one in my apartment complex with an air conditioner visible from the street).
So I came up with this arrangement. The outer box you can see allows the air conditioner to draw in ambient outside air from around the edges.
Not visible is the interior extension box in the center attached without gaps to the air conditioner that allows the air that's been heated by the radiator fins to be blown out the window in the center, with minimal recirculation of hot air.
I've also moderately insulated the outer box with a couple of layers of thin packing foam sheet, to help minimize outside warmth transmitted into the room. Likewise, I've insulated the inner exhaust tube (box) to reduce heat radiated to the intake air.
It's super effective!
Note: I have a drip pan under it, but it seems that this Frigidaire* air conditioner is designed to pick up some of the condensed water and splash it around in the vicinity of the radiator fan, eventually evaporating it. Perhaps as a result, very little water collects in the drip pan.
*-(Frigidaire 12,000btu model FAC127S1A)
P.S. Hopefully this posts the right photos. There seems to be a misalignment between what I click on in my gallery and what gets shown).