Looks awesome. I cant find my megadrive scart/phono cable so ive just added an audio out jack to my megadrive 2. Its not a prosound mod (search for crystal clear audio mod megadrive) but its good enough. Theres space to put an 1/8 inch jack at the back left or right, although the right might need some extra plastic removed. I opted for mono due to lack of spare interface channels.
http://www.mmmonkey.co.uk/mega-drive-2-av-port-pinout/
Remove all screws to get the plastic casing then metal ground shield off, then the screws either side of the cartridge port in order to lift the pcb out of the case. At the underside of the av port solder a wire to one of the 4 larger tabs, this is your ground cable. Then either solder one cable to pin 6 for mono or a cable each to 8 and 9 for stereo (see link). The cables can be threaded through the pcb mount holes then the pcb and ground shield screwed back on. Mount your jack to the case, careful to place it so the contacts wont be touching the ground shield-i drilled mine in the top half of the case. Solder the wires to the jack(s) and close the case