An apparatus includes a housing, a cable guide, a first cable, and a second cable. The housing is coupled to a shaft of a medical instrument. The cable guide is coupled to the housing, and defines a shaft opening into a passageway defined by the shaft. A first guide groove and a second guide groove are defined by the cable guide, with each of the first guide groove and the second guide groove being splayed outward from the shaft opening. The first cable is routed within the first guide groove and through the shaft opening, and is configured to slide within the first guide groove. The second cable is routed within the second guide groove and through the shaft opening, and is configured to slide within the second guide groove.