Have you ever used one of these cement mixing tubs before? They are made of an indestructible black plastic and come in two sizes that I've seen. This one is 25" long x 20" wide x 6" deep, but there are larger ones, and they can be purchased at a home improvement store. This one makes a great fabric dyeing container because the bottom is flat.
When my boys were little my sister worked at Lego, so you know we had thousands of Legos in the house. We kept them in these tubs which very easily slipped under the bed for storage.