What it really sounds like is that your current AC isn't working right and
the house isn't insulated/vented well enough to keep the heat from coming
in.

It's a one zone system for two floors, and so the upstairs is warmer.

If all you want is a bandaid, why not just install a window fan. Like you
said, the night air is cooler, so that should do the job.

How I would install a window fan in a casement window? The portable AC
could be installed using a piece of plexiglas, but not sure how a window fan
could be installed.

J.