I recently bought a portable air conditioner (This model: https://www.amazon.co.uk/dp/B0CP42SSZD?psc=1&ref=ppx_yo2ov_dt_b_product_details ), and was having some trouble with keeping the exhaust air from coming back into the room.

The unit does come with solutions for this, but it's primarily designed with sliding windows in mind, as they include a PVC “gasket” that fits into the gap in the opened window, and an exhaust collar (the white object in the photos) that clips into the gasket. This would work great for sliding windows, but not at all with outward opening windows. 

They also include a fabric bag for non-sliding windows, which is secured by velcro with sticky tape on the back, and the exhaust pipe goes through a zipper in that bag to vent outside. This solution doesn't really work, as the mass of the exhaust pipe/collar keeps pulling the bag off the window, and the exhaust pipe alone just does not stay put in the window gap, no matter how much you deform it.

So, I created this model to clip into the exhaust collar. This assembly can then be pushed through the window gap, making everything stay put. 

I'm now getting consistent cooling, and no issues with the window bag.

I was having some bed adhesion issues, so I turned the initial layer head movement speed down to 25mm/s. I also printed the sparse infill in the Gyroid pattern, and added an outer brim of 5mm to help with the bed adhesion.

I've included the f3d model files as well, in case anyone wants to tweak it. The one I've printed fits almost perfectly (the walls are fairly thin, and can be slightly loose on the long straight edge)

Edit: It turns out that this might fit a whole lot more portable A/C units that the one I have. I looked at a few pictures on Amazon, and it appears that other brands use the exact same ducting system. I can't guarantee it, bit if your exhaust duct looks like the white item in the pictures, it's possible that this mod will fit it.