Can I use a portable air conditioner with a casement window?
The easy, but ugly solution is to cut a sheet of plywood that will fill the entire opening, and cut a hole in that to attach the AC exaust hose.  Or cut the plywood to fit in beside the exaust hose, whichever is easier.

Then insulate and seal the crap out of it with foam boards and metal tape.

I would recommend avoiding portable AC if you can get one that will fit the window though - I have 2 of them, and they do not work as well as one that sits in the window.  They take more power and take twice as long to cool the room.

Depending on the window, you might be able to easily pop the window out of the frame to fit a normal window AC, with plywood to fill the rest of the opening.