I will describe the modification for a replacement oscillator in the modem bay, since that is the modification I have done twice. Anything with three isolated parts of copper will do. It is just to provide for a place to connect both the wires to the replacement oscillator and the wires to the on-board oscillator. Some isolated wire to run from the replacement oscillator in the modem bay to the small piece of pcb. Some ordinary glue.
Construction Take the small piece of printed circuit board.
Solder three long isolated wires to the copper pads. Cut of the pins of the replacement oscillator, leaving about 1 mm of pin free. Solder the isolated wires to the appropriate pins of the replacement oscillator. Arrange the wires carefully over the Duo pcb. Now glue the pcb on top of the on-board oscillator using ordinary glue.