"Chris" writes:

My question is: could I install the AC on the lead of the attic access, with
the face down. In that way the ~exterior~ would be the attic and the AC
would push down the cold flow air. The lead for access to the attic is on
the wardrobe, which is next to my bedroom where I need the AC.

A window AC unit will only work properly when installed as designed.
This generally means with the bottom almost horizontal, actually tilted
a few degrees towards the outside. That's because gravity has a large
influence on the flow of liquids, and there are three liquids involved
that would be affected by any other orientation:

1. Water.
Water condenses on the evaporator, where it's supposed to be collected
by a pan under the evaporator in the "indoor" portion, then run to the
"outdoor" side of the unit (that's what the few degrees of tilt is for).
Once in the outdoor side, it's often splashed onto the condenser by the
fan. This cools the condenser, and makes the unit more efficient. And
if there is too much water for the unit to handle, it drips off the
outside of the unit.

If you mount the unit face down, the condensate will drip back into your
room.

2. Refrigerant
The refrigerant turns from gas into liquid in the condenser. Once
liquid, it flows to the bottom of the condenser where the liquid is
then fed through the capillary tube and on to the evaporator. If the
unit is mounted face down, the liquid refrigerant will likely pool
somewhere other than the intended "bottom" of the condenser.

3. Oil
The sealed compressor housing includes some oil that lubricates the
compressor bearings, piston rings, valves, etc. If you mount the unit
face down, the reservoir of oil inside the compressor will move to
somewhere it's not supposed to be. You could get poor lubrication, or
too much oil in the cylinder during compression. You might destroy the
compressor in short order.

I can not install it on the existing windows, different model and size.

Sell the existing one, buy new one that will fit.

Dave