The rubbermaid stock tanks seem to be the popular sump choices. They come in 50, 70, 100 and 300 gallon capacities, but the smallest one is 28" high, 23.5" wide and 48" long. I don't think that would fit under your tank easily.
I have seen some of the smaller storage bins used (and used them temporarily myself) that are available at Depot or even Big Lots. The issue with these is the plastic does not hold its shape and the sides bow out when filled with water. That said, I have never had one fail on me.
When I was moving my tank I doubled up some 40gal bins but they still bowed. Perhaps add a frame to keep it from bowing?