I've been flying for over 40 years, and I still use my fingertips for balancing. It's as accurate as you need it to be. IMHO using a fancy mechanism for balancing is about as needed as buying a plastic "Rubber Band Puller" so you don't have to use your fingers.

But as to your statement:

What's wrong with putting the battery behind the servo tray? Heck, if you need to, put an extension on it and stick it in the tail.