Figured bass: stacking space instead of digit

I once did this by accident (so I know it's possible), and would very much like to do it again. What's the trick

I want successive figures

5
4 3

i.e., the 4 goes to 3; there is (and ought be) a "space" stacked above the 3. I did it once by accident; how do you do it on purpose?

Thanks.