A piece of conduit with the end slightly crimped to form an oval, and crimped down far enough to accept the blade. A weight at the tip will help balance the sword on the hip (often the swords are hilt-heavy). The conduit also helps make the metallic "shwing!" when the weapon is drawn. The exterior can be spray painted or wrapped in leather.
I've also seen golf bag tubes used in the same way, esp. for wider blades.
for really wide blades like Roman or Medieval, two boards with groove hollowed out for blade, glued together and wrapped with string or leather!
Ideally, the sheath can be easily detached and discarded, since fighting with a sheath swinging around loosely is a problem with both safety and aesthetics.