hmm, I may have calculated too much then. If you can get by with a depth of 2" then I would need what, 10-11 bags of the 9L? Dimensions of the tank are 60X24 (roughly).
With shipping that's about $410.00 - 440.00 with shipping making up a third of that cost.
Hence my thoughts about using soil/clay with a cap.
I do appreciate your feedback