I highly recmmend getting the High Step topper (as JT says, if you have the regular club step already, the topper will work with those risers). The Fanny Lifter, which works well as a step-up box (and, by the way, as a laptop table, which is how I've used mine for the last 3 years!) is not as stable as the High Step, and it would be too easy to catch your foot on the edge of it and kick it out of place when doing some of the 'fast feet' drills. It's also not meant for impact, and doesn't give as much as the high step.
Another alternative, for many of the cardio moves, is to just turn your step vertically (so when you look down, the narrow end is facing you) and work off the end. There is only one move, I believe, that would require modification (a move that has you moving around the high step on all 4 sides). This is what I did before I got the topper.