Taking a large garbage bag, I put the tray inside. Turn it over and tape it down so it doesn’t move. Make sure to leave room on the top side for the paint.
It worked very well. The one caution I would add, is do not leave your paint in the tray to use later. With plastic, as with tin foil, you will get acrylic paint peeling off the tray and adhering to the roller. You may get bits on the walls.
Use it quickly and put only as much as you need into the tray. It works very well.
Clean up was easier too. Just pull off the tape and fold the bag over itself. Discard.