Yes plastic is ok to use as long as they don't chew on it. Eventually you would like to switch them to a metal one or a lit of people use a glass baking dish.
If it's plastic, they're going to chew it! If not today, then tomorrow. I had a plastic tray in my first chin's cage. She didn't touch it for 6 months then she ate a huge chunk. Glass pyrex comes in many sizes at Walmart, many owners use those.
Might take a little while for a response, she said shes not on here as much as she should be lately, but you can PM AllMyChins. She sell metal corner litter pans.