Auto supply stores have a trim attachment tape. You can also use adhesive mounting squares sold in hardware stores. Be sure to apply two coats of Ambroid cement first to the balsa to give a smooth, sealed surface for the tape to adhere to.

Another method is to wrap the servo in a piece of mylar package sealing tape and then stick it in place with GOOP cement. The tape will make it easier to remove later. The advantage of this method is that the servo will be held securely with no movement under operating load. The thickness of the foam mounting tapes will allow for some rocking of the servo which reduces your control accuracy a bit.