Big Bike Parts has a good inline control. The J&M units don't work well, and are overly complex for the task, so I would never design a unit like that. I have plans for a fader, balance and 'mono' option unit that will eliminate the need for front/rear volume controls, but it's a year away at least.
 
PS: This forum is not for support, it's just for customers to discuss among themselves, so I am not guaranteed to follow it closely. If you have timely stuff for me, email or the support feature is best.
 
Ed