>Kathryn, I do have a step but I only have 6 risers. If I put
>three under each side, it probably won't be enough for leg
>work, right? I guess I could put it on a few 2x4s! LOL!
10" is about the lowest I would suggest for the leg presses, but it's sufficient, especially if you 'dip-down' (squat down) at the bottom of the move and push off from the heel on the way up. 10" could be effective for you, especially if you are short (around 5"). Also, using heavier weights can add intensity to the moves. It's a good starting point, and if you get to the point where you feel it's not effective, maybe you'll have saved enought for a high step by them.
By the way, since you have 6 risers, you only need to get the High Step topper (no risers), since you can use the risers that you have. That would give you a maximum of 16", which is more than enough for step-ups, and even high enough for sit-and-stands, unless you are tall).
I wouldn't recommend propping the step up on anything else, it could be too unstable.