I have seen threads about making a cable to convert a stereo signal to a mono one but all that I've seen end in a mono plug. An amp I'm looking at has an auxiliary in port but I want to use one channel for a pair of speakers and the other for a small subwoofer. This is to go to a backpack boombox setup. Any advice would be wonderful.

 

You don't need a stereo->mono lead, you need a stereo->dual mono leads. Sometimes called a "Y lead".