Food service basins, trays, whatever you want to call them, are great. I got mine for about $6 each at Smart & Final. They're about 13x17" and about 5" deep and are made of a heavy grade gray plastic (about 18% gray in fact). They're normally used to bus dishes in restaurants. I use mine to wash prints but will be getting more. The plastic is pliable enough to work with and won't shatter so it might be easy enough to convert with hoses, etc. into a proper washer...

S