Best way to run a simple prize draw in Marketo?

Hi everyone, there might be a very simple answer that I'm missing (it *is* Monday morning after all...) but I'm hoping to build a quick campaign to split a list for a prize draw we're running.

From a list full of all entrants,  we'd like to segment an exact number of users into a 'winners' list and the remainder to an 'non-winners' list.  My problem is that I seem to be able to only select a % random sample. Seeing as we don't know how many entries we'll have yet, the % option won't work (nor do I love the idea of having to wait until all the entries are received to be able to add the % random sample required!)

Any ideas, Marketo community?