What material can I use as a temporary window pane?
You can buy big sheets of plexi at big-box hardware stores, or online cut to size. A simple websearch for "plexiglass sheet" turned up a large number of wholesalers.

However, the cost may surprise you; a 36" x 48" x 1/4" sheet was $100 at the random place I checked.

Still, afterward you'll have a huge piece of very durable plexiglass that you could use for something else.