I think you could warm a lot more liquid if you use a well isolated container. I my self have a setup with a pizza delivery box that I made waterproof. I don’t use milk in it but the water stays at temperature for hours after the ANOVA is removed. In a normal container heat is lost at a much higher rate. Because the box is so well isolated I can use more water than I could use with a non isolated polycarbonate container.
Making the pizza delivery box was a lot of work. You could also use a cool box. Conversion of these for sous vide instructions can be found on YouTube. I don’t know if this system would work for two times the liquid as normal though.
In general I think polycarbonate boxes are not very energy sufficient.