If it's not too late, I would glue rails to the inside of each fuselage side before gluing the fuselage together. On top of these rails you can either glue a plywood plate for you servos or a pair of rails that the servos fit between.

It's easier to do this before gluing the fuselage together because you can more easily measure and get the rails parallel. I would use 1/4" x 1/2" hardwood of some sort with the 1/2" side glued to the fuselage.