I wanted to make this badge holder as a surprise for a very punny friend of mine.  The first one, on the right, trapped the cardboard on all four sides, and it eventually became clear that there was no way to get the cardboard inside without creasing it.  The final design has a slot along the top, through which the cardboard is inserted; the strings keep the card from coming out.  Notice that the string holes had to be moved just a tad.  It took three tries to get this simple design right!