I've never used a carboy brush, just a sprinkle of pbw some hot water and a rag. Swirl it around till clean. But to each his own.
A one gallon plastic measuring cup was one of my most beneficial early purchases.
https://www.amazon.com/gp/aw/d/B004QNWUWS/ref=mp_s_a_1_4?ie=UTF8&qid=1498492958&sr=8-4&pi=AC_SX236_SY340_QL65&keywords=1+gallon+measuring+cup&dpPl=1&dpID=41FjAMR65WL&ref=plSrch