I let others reply, but my experience and advice would be to put them in a case at a public festival. A knife show has knife people and somewhat of a bit of order to it. A public fair or festival is not the place for sharp things out in the open. Loosing a knife to a thief would be a minor worry. Having someone cut would be a much likelier situation.
The closest thing I ever saw to a non-case method of secure display was a weighted bar that sat across the knife handles. It was at a knife show in NC, and the person had out about ten really nice large bowie style knives. To my observation, the bar was a 1/2" round bar of steel with a rubber coating on it. It was set over the knives so it rested on the handles just above the guard. There was some sort of blocks of wood on each end to keep it in place ( and maybe lock it down?). The bar could easily be lifted a tad and a knife slid out, but it also clearly implied without words to anyone looking at the knives "DO NOT PICK UP KNIVES". There was descriptive label next to each knife with info and price.
Kevin,
The last time I did a show in Hampton (never went again - nothing but crazies and goth types there), a mid-20's fellow came up to the table in a gi type tunic and obi. He picked up a large tanto to look at. Without asking or saying a word, he made a bunch of kung-fu moves in the crowded aisle, complete with Bruce Lee yells. At the end of his "kata", he pulled the sword back to his left side in a ferocious looking "Ready Position". As he did this, he caught the tip on his gi and cut right through it. Thankfully, he barely nicked his skin, but it made a several inch long oozing slit that could have been serious if it was 1/4" to the right. The knife was taken from him by a large military looking fellow who observed all this, and shortly after the nut job was escorted from the building by the police. He easily could have injured or killed someone.