Huber's suggestion with a clipped 6 arm servo wheel is the way to go. It worked great on a trainer of mine as well as with a Goldberg Cub both using std. servos. The amount of differential is fixed but is close to ideal anyway.

The advantage of differential aileron throw is that by getting more throw from the upraised aileron and less throw from the lowered aileron, you develop less lift and less drag on the outside wing and much less lift but more parasitic drag on the inside wing. This results in less adverse yaw during turns. With strip ailerons, it can pretty much cancel any adverse yaw in turns as there is relatively little adverse yaw with them anyway.

A well designed sport aircraft will have a good balance between dihedrahl and fin and rudder areas that results in a correct amount of roll/yaw coupling and reduces or eliminates the need for any rudder input in normal turns. On these aircraft, differential throw (other than any specified in the plans) will offer little or no benefit.

But differential aileron throw can be very useful in scale aircraft that would otherwise be much more demanding of good rudder coordination during turns and aileron inputs. Broad chord ailerons at the ends of the wing as on a Cub are good examples of ailerons that will cause a large amount of adverse yaw.

Nearly all trainers have a wing with a lot of dihedrahl angle. Yaw on an aircraft will cause the inside wing to advance relative to the outside wing. The advancing wing, if it has a large dihedrahl angle, will experience an effective increase in it's angle of attack and will develop more lift which will in effect decrease the roll rate by attempting to roll the aircraft upright again. With a properly proportioned vertical fin, the aircraft will be overly stable and essentially self righting in roll. (with a large enough rudder in proportion to vertical fin area, the plane can make coordinated turns on rudder alone as is done in polyhedrahled sailplanes) This stability is a good trainer trait but can be a little irritating when your skills improve and you want try some aerobatics. On an overly stable aircraft, differential throw can increase roll performance by again reducing adverse yaw and reducing the tendency of the inside wing to generate additional lift due to the unwanted adverse yaw.