There are many ways to arrange springs, wood or metal, to hold in the cards. A steam bent thin wooden spring would be simple and elegant. It would hold them in by friction.
A wedged lid could also be used to hold in the cards. Without a hinge, the wedge would be easily dropped or lost. This is avoided by putting a pin through it, which slides in grooves machined into the top and bottom plates. Although it's a little bulky compared to the others, it is repairable and looks unique. The top and bottom plates would best be made of sheet brass or stainless, and the rest made of hardwood. The side edges are convex to be gentler on the hand, and the bottom is flat so it can stand on its own.
I think stainless would look good with ebony or black walnut, and brass with cherry. The side pieces and wedge could even be made of micarta, for an especially modern appearance.
The third idea could probably be made the slimmest. For an even lighter design, the hinged lid pictured below could even be removed entirely, and the sprung catch could hook onto the corner of the cards themselves. This design would probably be the most fragile of the three.