To All Business Card Display Gurus...

I think the business card display is a great biz idea.
But I just had one problem.
If you are going to make your own board , How should one design it to keep the people from taking the cards out and putting their own in (without paying)
Would putting a plastic cover atop work or what....?

I think if someone could figure that one out the biz card display center could be of great service for the local community plus making the business display owner ALOT of money.
What do ya'll think??