Plant of The Month: 2012 - Creme de la Creme Photos: Dec 2016, June 2017
not sure how many gallons it is, but I bought rubbermaid containers that are about 6 inches high that fit under a bed for clothes storage. They are about 18 inches by 32 approx. Kid used them for storage at college or I'd measure one for you. I doubt they hold 20 gal though.
I bet it would be tough to move around though with water in it.