Gary I have several planes with tail mounted servos whats worked for me was to just re enforce the sheeting with light ply where the servo screws are.
using med.CA to glue the ply in,behind the sheeting,if you want a recessed servo then enlarge the hole enough so it will fit flush, then just glue the ply in so the servo can screw to that.
for the surface mount then cut the sheeting to the size of the servo body,again glue the ply doubler behind the sheeting to strengthen the screw areas the ply doublers should be aprox 1/4 to 3/8" over each side of the servo,and 3/8-1/2 " wide.
then after you tap the screw holes harden the threads with thin CA,let dry and mount servo, other than that not much else needs doing