Hi Bob,
You may be able to get away with a jigsaw using a blade for plastic. Just depends on how thick the box is. If the box is too thin, a jigsaw can catch and tear the plastic.
I made a cutout in a plastic box for electronics displays, and the plastic material is polystyrene, which means it can crack if not cut carefully. I used a cheaper Dremel knock-off with a cutting wheel, and it worked like a charm. I imagine that a Dremel/cutting wheel will work on the material that cement tubs are made from. Thin plastic can be a pain to cut with anything reciprocating, but the high-speed rotary motion of the Dremel enabled a clean smooth cut without cracking on the box I mentioned above. Just make sure the box is held fast - you don't want the workpiece to slip from you when cutting! Might be worth a try.