I’ve been asked about how I programmed the bootloader so I’m putting the doco here. For all my AVR work I use a bus pirate v3 with the stk500 firmware. I have a cable I made that adapts the bus pirate to the 6 pin AVR ICSP connector.
For MakeyMakey I made an adapter cable to convert the 6 pin AVR ICSP connector to the MakeyMakey header. I prototyped the cable using individual 0.1″ socket to pin cables and then moved them to single 2×3/1×6 housings to make the cable permanent.
The MakeyMakey schematic decodes the header in two parts
A random ICSP image from google images.
The pin mapping is
orange MISO 6x1-6 2x3-1
yellow MOSI 6x1-5 2x3-4
green VCC 6x1-4 2x3-2
blue GND 6x1-3 2x3-6
purple RESET 6x1-2 2x3-5
grey SCK 6x1-1 2x3-3