Colin,
Here is an easy, no way to mess up, way to do it. You already have the cable with the correct pin outs on it with your excisting serial cable, your db9 M/F sides. All you need is a 25' chunk of cable to use, all serial cables are the same, some just pin out differently. So buy any 25' serial cable from somewhere like CDW and cut the ends off and strip the individual wires down about a 1/4 inch. Then take your existing serial cable and cut it in half, strip all those wires about 1/4 inch, then just match up all colors and wala there is your needed length. Once the wires are connected I would solder and then use electric tape to connect. You cant go wrong because you already have the correct pin out, your just matching wirecolors at this point. I have done this many times.
Terry