How to safely install a window AC unit

You could build a wood frame that fits in the window with an interior hole large enough to accommodate the AC unit. This also has the advantage that you won't have to hold the window open, while juggling the unit into place. You can install the light weight frame, then slide the unit into place.

I built one when I lived on the second floor of an apartment building, because I was always afraid I would drop the unit out the window while installing it. I used foam tape around the exterior and interior of the frame, to make sure I got a good seal around the window and AC unit.

For a unit that only vents through the window (not sits in it), you could build a smaller frame with a port to allow the vent to pass through. Filling the interior of the frame with Lexan will reduce the amount of wasted window space.

(You could always use smaller lumber for your frame, if you want it to have a lower profile. The frame size in the image is just an example, to illustrate the idea).

Since it sounds like it might be difficult for you to get to a lumberyard to build your own solution, you might be better off using a pre-made product. AC-Safe, makes Universal Air Conditioner Supports that might be useful in your situation.

The system requires you to screw the platform into the window sill (which may or may not be possible in your situation), then simply rests against the outside of the building to support the A\C unit.

I've also seen brackets that mount to the bottom of A\C unit itself, and offer support against the outside of the building (but I was unable to find an example online. I think Thermwell makes them).