I've developed a composite video mod for the Sega Master System II that doesn't require any bodge wires or holes drilled in the case.
By removing and changing a few resistors and capacitors to match the SMS1's audio/composite video output circuitry and installing a custom PCB in place of the RF modulator, you can get a high quality A/V signal with no "jailbars" or noise.
More information (including complete installation instructions and PCB Gerbers) is available on my website.