Portable Air Conditioning Venting

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).

It's possible but it will be creating a situation where your a/c is fighting against itself, because you'll simply be redistributing the heat inside your house, instead of putting it on the outside.

Why? Because the warm air will creep back into the room you were cooling, so the a/c will be acting like a big do-nothing machine - to a degree anyway.

Mold isn't going to be too concerning, however, because you won't be taking moisture out of the one room and putting it into the other - the moisture in the air in the warm room may increase a little (because warm air can hold more water vapor than cold air, so if moisture is there, it'll get picked up) but not to the degree it would if you were, say, venting a dryer into that room, where you'd be introducing lots of NEW moisture.

All that said - I do not recommend this idea. Better to create a vent/opening (properly, of course, but that's a different question) in the outside wall to allow the warm air to go where it is supposed to.