I feel a way better way to increase the stability demands of an exercise is to narrow the footprint of your base, rather than using some unstable surface gadget. You can narrow your base of support simply by bringing your feet closer together. From that position you could go to a split stance with some exercises. Or even go to a single leg stance.
A couple of weeks ago I showed you how to make the plank walk up a bit harder by elevating your feet. Here’s another way. Go to a single leg plank walk up. You’ve just narrowed your base of support making the stability demands and the overall exercise much harder.