MinnFlyer has a great idea. But, it that doesn't work here's one more thing you could try...

Take a dremel and cut the elevator in half. Wood, wire, and all. Remove the center section so now you just have a half on each side. Attach another control horn to the other half of the elevator and run another push rod to the servo. This will give you adjustablity for each side so you can get them both perfectly even.

But still, try MinnFlyer's idea first.