We recommend using plexiglas.
Casement window portable air conditioner vent.
This would also work if you want to install an air conditioner in a sliding window as well.
It works by taking in air from a room cooling it and directing it back into the room venting warm air outside through an exhaust hose.
However any type of material that will keep the hot air out and the cold air in will work.
You can see how one homeowner did it in the video below.
A portable a c unit also known as a stand up air conditioner is a versatile alternative to central air conditioners and window units.
Although most portable air conditioners come with a traditional window kit you can absolutely fabricate an insert to fit into the casement window allowing you to vent your portable air conditioner.