You really just need a large plastic bin with a flat bottom. (Contoured bottoms are a pain to scoop and even worse to clean.)
I chose the Ikea Samla
31x22.5x7" clear under-bed storage box. It's big enough for a tiger cub or 30 pound dog. It's squarish shape allows for numerous spins and sniffs prior to use, the walls are high enough to retain litter and prevent animals from squatting over the side, and it's a departure from the narrow under-bed options from Rubbermaid and many other bin brands.
You'll also potentially need: