you can get about 7.5 gallons of liquid per cubic foot. 2000 gallons would take 267 cubic feet. Lenght times width equals square feet and then times height equals cubic feet.
Two tanks side by side with some wiggle room to get them in and out may get you a pad with an inside size of 9' by 13' which equals 117 sq. foot and divided into 267 would get you about 2.28 feet high to hold 2000 gallons.