Pinterest has some options on how to do this:
When my girls were little I used a shallow Rubbermaid container as a water play table. You can glue it to a small little bench to make it higher.
Creative and simple
You can use a shallow storage tub, you can use E6000 to secure any toys (like a little water windmill type thing) to the bottom of the container. If you want it to be portable, glue some Velcro strips to the bottom of it and if you have a little table/bench you want it to sit on sometimes, secure the other part of the Velcro strips to it, then you can take it with you or move it easily for emptying/cleaning.
If you can't find any the size you want, you can get epòxy putty I think it's called or caulkiñ used to seal fish tanks, you can use either to attach 2 or more smaller containers together to make a large container