Which material for a sink bucket?
#1- Plastic Buckets are cheap. Use one 'til it is no longer usable, buy three.  (My favorite are the Lowe's buckets. I have put all kind of nasty stuff in them.)
I lived for many years with a water jug on a shelf above a double sink with  plastic buckets under each bowl (and a camp toilet in the bathroom).  The key  to remember is LOOK UNDER THE SINK to see how full the buckets are.
A small galvanized trash can would hold more liquid. I am not sure how water tight they are. I have had ones that held water. Maybe a stainless steel one from a restaurant supply house would be a good investment.
A galvanized bucket would be a good choice.

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