Where are you mounting your battery? Normally if you mount it close to your seat post and use the Y cable there, you will not have that much extra cable left over from the Y or the extension after running that to the front. It almost sound like you are mounting your battery pack on the center of the top tube.

Regardless, if you do need a 8" extension cable and worry about cutting and soldering, then I suggest you get another Y cable. Those are about 12" long. Use only one of the two connector as a short extension. Tape the two cable of the Y end together so that the unused one will not flop around.