I needed a DC power extension cable for a BT Home Hub 5 and found that a 5.5 x 2.5 mm extension cable fits on the female end but the male plug that goes into the socket on the back of the router would not quite fit as the internal pin in the socket is a little bit bigger than 2.5mm. So I used a 3mm drill bit to widen the hole in the male plug and this seems to work.
