#5 (PP) polypropylene

[gutreactor]

This is the best plastic for mixing lye. To my surprise, my spouted buckets from the local hardware store are #5 polypropylene. If you look for one be sure to check the bottom. I have one that has raised areas in the bottom which wouldn't be good for soap and one that's just concave on the bottom. It's 9 QT (8.6 Litre).
Found a #5 polypropylene measuring pitcher online - Accu-Pour Measuring Pitcher. It comes in 8 oz up to 128 oz.