An apparatus for guiding an elongated flexible instrument, includes a first plurality of linkages forming a first side of a channel of a variable-length support assembly, a second plurality of linkages forming a second side of the channel, opposite the first side, a third plurality of linkages disposed between the first and second plurality of linkages and forming a third side of the channel, and a fourth plurality of linkages disposed between the first and second plurality of linkages and forming a fourth side of the channel, opposite the third side. Each of the first, second, third, and fourth pluralities of linkages are separable from each other to transition the variable-length support assembly from an elongated configuration to a compact configuration.