We have these easels at work.  the one big design flaw was nothing held the top and signs would blow or fall off.  I designed 2 kinds of clamps.

The first attaches to the legs and has a lip that hangs out to grab top of sign.

The second uses a hole and clamp on the top, originally designed for a cart holder to attach, and sticks above the top of the easel.  As built the top is 6 inches long but I included the CAD so you could lengthen or shorten the top piece to fit your needs. 

Just a quick useful item if you have these easels