Glue thin plywood to the underside of the balsa, and attach the servo with servo tape. Two-foot aileron extensions are the perfect length. I used the holes I had originally cut for the cables, except for the two closest to the servo. I removed the connectors from the extension and the servo so I could feed the wires thru the little holes. Lock the connector together with a small wire-tie to keep them from accidentally disconnecting.
Some people use shrink-wrap tubing instead.
As you can see, I covered over the servo. If you need to get to it later, just cut the covering away and apply a patch as I have done here. I set the aileron differential up using my JR-642 programmable transmitter. I also set up spoilerons by setting up the transmitter for flaperons, and setting the travel in the opposite direction so the flaps extend up instead of down. I understand that they can save the day if you get caught in a strong thermal.