How To Better Incentivize Labor Economists?

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The annual conference that I organize with the Institute for the Study of Labor in Bavaria begins Thursday. Each year we receive about 150 submissions, and pick 24 to fill the available time slots.  Typically we’ve had one or two withdrawals, but this year we have five. If I had known this, we would have accepted more papers—the conference works best with 22 or 23 papers. Is there any way to solve this problem? I could accept more papers, but the program would be too long if nobody withdrew. I could require authors of accepted papers to post a bond, perhaps $250, forfeitable if they withdraw. While that sounds very economic, I bet we would get fewer submissions—posting bonds for conference participation is not part of our culture—and possibly even lower average quality submissions.
Of course, I punish those who withdraw by disqualifying them from the conference for the next few years. But other than that, I see no solution.