I have two coolers, a small one that is 9.5 litres for smaller stuff and a larger one around 24 litres that I can cook a whole (1.7kg) chicken in easily. The larger one holds a lot of water so it takes over 20 minutes for the Anova to bring it up to temperature, the smaller one only holds around 1/3 as much water so the Anova can get it up to temperature in about a third of the time. Either way I'm using an insulated container which seems like a good idea to me.
My smaller cooler (we call them chilly bins) is 10 quarts or 9.5 litres, it is a Rubbermaid brand that I bought on a close-out sale.
My larger cooler is one I had lying around here.