A few years I found curbside this rigid plastic kiddie pool that is about 7 inches deep. In this pic it is serving as a nursery for my potted Louisiana Irises....they are swamp plants that need very damp soil.
At other times I have used it to deep soak potted plants that had dried out. Today I packed it with 1 gallon pots and planted in each a division of Allium canadensis, a native garlic relative ranging from Texas to Florida as it too likes very damp soil.
Something that works good for this are those clear plastic storage tubs that many people throw out when the lid cracks. But the kind of plastic they are made of gets brittle in sunlight in six months or less. Sometimes when I am dumpster diving behind restaurants to get scraps for my chickens I will find a bus tub. They last a long time out doors, are easy to carry, and hold about 4 inches of water to soak plants in.
Always remember the dumpster diver's motto: "Peek and You Shall Find!"