Hi @ageorge,
A warm welcome to the Bunnings Workshop community and thank you for your question.
Unfortunately, there is no simple way to install a portable AC vent to a window like this. You're going to have to build something custom to fit the window.
You'll have to start by removing the flyscreen frame and measuring the width of the channel it sits in.
You can use an aluminium channel that is the same width as the channel the flyscreen sat in to create a frame for a sheet product to sit inside.
You can then use a sheet of plywood or acrylic that is the right thickness to sit in the aluminium channel to block the air from outside.
Most portable air conditioners use a plastic plate that can be expanded vertically but not horizontally, so your new frame should be the width of your flyscreen frame minus the width of this plastic plate.
Cut your plywood or acrylic and aluminium to size and glue the channel around all four sides of it with Liquid Nails. Keep in mind you'll need to cut a notch in your frame to accommodate the window winder.
Allow me to tag @JoeAzza, @TedBear and @DIYGnome to see if they've got any ideas.
Let me know what you think.
Jacob
